Best Of

Using 2 boilers with Steam, you will most likely have both boilers start at the same time and produce enough steam to get the pressure up to about 3/4 of the operating pressure then have a boiler drop out.  The other boiler will keep operating until the call for heat is satisfied.  If the one boiler can not keep the pressure high enough because the load is greater (because the outdoor temperature is lower and the radiators can't keep up with the amount of heat needed in the rooms) then the second boiler will cut back in if the pressure drops.

Another way is to operate only one boiler when with an outdoor temperature sensor in series with the other limits.  

The best way is with a Heat Timer staging system with a little more logical inputs that take into consideration several inputs like outdoor temperature, boiler pressure, and cycle timing.  

Two boilers are usually more economical to operate and sometimes less costly to maintain since you are using lower price boilers and parts.  Sometimes there is an economy of scale with performing two maintenance procedures without moving your tool box and truck to another customer so the actual maintenance per boiler is less expensive.

With Hot Water the answer is easier.  It is definitely more efficient to use two boilers.

The steam rises so it can be supplied from the bottom of the radiator. most recessed style radiators have the tapings at the bottom. most convector elements have some sort of end tank, either a separate part or cast in to a cast iron version that the supply connects to. If it is one pipe the valve needs to be arranged in a way that the condensate can drain through it, that it won't hold an appreciable amount of condensate in itself or the pipe.