Multiple temperatures on single zone with single circulator pump?

Actually no, I would not do that if the OP has an unlimited budget. I would suggest a Taco Iseries valve that has a motorized head with outdoor reset capabilities. 3 way ODR mixing valve 1/2" That way the ODR from the boiler can be set for the high temperature zone(s) needs and change the boiler water temperature based on the needs of that higher temperature heat emitter. 125° boiler temperature when the outdoor temperature is 55° and 180° water to the radiators when the outside temperature drops below zero. The Taco Iseries valves are discussed in this video at about time stamp 4:20.

But the actual valve is discussed in more detail in Part 4 of the series, here

However looking up the price of that Iseries valve that can be used to reset from an already reset higher temperature heat emitter zone, it is a little pricey for someone that is interested in keeping costs down. If you look at the OP's original design, to save on additional costs of things like a bronze or SS circulator, they were asking if the diagram would work. I believe that diagram actually has a mixing valve that is already on the job. That mixing valve is a fixed temperature or manual mixing valve that was used to operate the 4 loops that were existing. When two of the four loops became unusable and unrepairable for reasons not disclosed, the idea of replacing the heat in the area that was serviced by the abandoned loops, could be heated by 16 feet of copper fin type baseboard with a piping design that will clearly not work at all. That design was the brain child of the OP with the idea that only one circulator might work, saving the cost of a second circulator and the associated control. With this in mind I gathered that adding more parts than the bare minimum is not in the OP's budget,

 So to answer your question about a fixed temperature mixing valve being the recommended option for the design using 2 circulators… My answer would be YES in this particular case.  Sorry to disappoint but I have not been given the option to spend someone else's money.

Although not an ideal way to use the existing ODR water that is feeding that room/zone with two different heat emitters that function completely opposite of each other. (High GPM High Temperature v. Low temperature low flow rate). And there is no “fighting each other”. The lower temperature on mild days will still go through the fixed temperature mixing valve and will cycle based on the room temperature controller (slab sensor/thermostat/etc.) The existing (no need to purchase another one) fixed temperature set at say 120° for those two remaining loops would only mix more of the 140° or 130° water and less of the return water, to get the 120° low temperature water. There would be some reset when the hot inlet of the fixed temperature valve dropped below 120°, which could happen.Then the cold inlet would stay closed and only the lower temperature 110° or 105° or even lower temperature water entering the hot side of the valve would be the only water out the mixed port of the valve.  So to put your mind at ease the OP will not need to provide boxing gloves to the mixing valve and boiler reset control to duke it out in the boiler room.  This will be a fight free design. 

I hope this helps you to understand why my design (drawing C) was not the most effective use of all that is available to make this system as best as it can be. I just wanted to let the OP know that the design (drawing B) was flawed and offer an economical solution. And the diverter valve idea will not work for the reason HotRod stated. one way in and two ways out. not a good plan at all. The design calls for two ways in and one way out. Definitely a mixing valve situation. Fixed temperature thermostatic or manual… must be two in and one out. I believe you will agree with that.

If only @inaun would come back and clear up some of these questions. I guess we will never know.

Most 3 way thermostatic need a 20 degree or more differential between hot inlet and mixed output, to maintain a stable outlet temperature. So the ODR could mess with a 3 way thermostatic.

Early on I suggested a 3 way manual mix valve. It would float along with the ODR temperature, called proportional reset. Some limitations with a dumb 3 way…since it is flow dependent, zone valves in the high temperature side would change the gpm flow rate, a dumb mixer could not respond to that condition accurately

But not knowing what SWT the radiant requires at design, the ODR curve would need some trial and error

Some examples from a 2003 article in PM ENGINEER by John Siegenthaler

Figure 2 with two intelligent mix valves

Figure 3 one smart, one dumb mixer. The crossover bridge on the mixed loop, with balance valves assures both loads see the same SWT.

I'm getting dizzy.

Back on October 5 @EdTheHeaterMan posted three of his lovely technicolour diagrams of this setup. A being the one in place (we think) and B and C being proposals.

Of these, only , with C two pumps wilt be controllable. I agree that Option B, with just the one pump, will work, but there is no way to control the relative heat output of the new baseboard radiation relative to the radiant. If you unwrap the diagram in a slightly different way, you find that the baseboard radiation is on what should be the primary boiler loop of the system ( presume there is a boiler circulating pump in here? We haven't heard much about it…) while the radiant floor is on a secondary loop. The problem being that mixing valve at the takeoff to the radiant, rather than that being just a T.

Use option C and treat the whole thing as a primary/secondary with a mixing valve or thermoostatic valve to control the temperature in the radiant loop…

I was kind of in the impression both expensive devices already exist.

If the boiler water can be in the range of 120F to 180F you want the automatic limiting action of the thermostatic mixing valve.

Cancel each other out ? They work in concert.

" I should have also noted this is just one zone in a larger system. "

Something in the rest of the system may need the 180F. So the thermostatic mixing valve limits the water temperature to the appropriate temperature for that section of the system.

I suppose if the OP wants to tinker and constantly make adjustments to stay comfortable. go for it. Plenty of room to experiment with hydronics.