2 floor, 2 unit home with two old oil steam boilers. Can I replace with 1 gas boiler, with 2 zones

Steam and zone valves don't work well together. But I had a church that had 3 zones using zone valves. one was on a 2" steam main and the other two were on 3" steam mains.

How large are the present boilers?

Unless the connected load is large enough for a burner that has high and low fire, it’s generally not a good idea. They are usually not made for anything below 500k btus.

In a steam system, the boiler output has to reasonably match the connected load. If you attempt to zone a boiler that only has a 100% fixed firing rate, it’s gonna be grossly oversized when only one zone is calling. You’ll have issues that can’t be easily corrected.

You’re better off just going with two properly sized and installed gas boilers.

I’d recommend the Burnham Max Steam.

Here is some interesting info on steam and zone valves

https://heatinghelp.com/systems-help-center/zoning-steam-systems/

I never met the dead man that installed the 1920s Smith one pipe steam boiler system in the basement of that church in Cape May NJ I used to work on. But it was originally installed in a basement. I could tell by the covered up chimney opening in the corner of the basement.  The boiler was close to a window that was obviously used to deliver coal to the basement.   At some point that boiler may have been under water since much of Cape May is below sea level and flooding is a regular occurrence for those buildings with basements.     

At some point in time (1940s or 1950s) the church upgraded the boiler to operate on oil heat and so a section of the parking lot was used to build an above ground boiler room.  You can’t have an oil burner and all the related electrical controls getting flooded out.  This must have been an interesting feat of engineering to move the basement boiler (all 16 sections) to the new boiler room that was above the steam mains in the basement.  There is a condensate return pump in the basement that was connected to three steam dry returns that dropped to the floor after passing thru steam traps from each return.  

So now the returning condensate goes into the return pump reservoir and gets pumped to the boiler thru a small tunnel that contains the steam main, and the return pipe from the condensate return pump, some electrical wiring from the thermostat(s) and power for lighting and the boiler, along with Hot and Cold water piping for a commercial water heater that is vented to the same chimney as the boiler.   

At the time of the boiler being moved there were some Honeywell motorized valves added to the three mains so the entire building would not need to be heated.  This was a very large building that included the main church building.  Then an annex was added that was 2 stories.  It was about as big as the original church building with two floors.  The first floor had the equivalent of 2 class rooms and 2 activity rooms.  The second floor was one large auditorium that was used as a basketball court at times and other activities that required a stage at one end of the hall.  There was a third area that connected the church and the hall that included a small chapel that accommodated about 40 people in the pews and a small altar at the head of the room.  Just behind the chapel was the connecting hallway from the main church to the annex hall.  The restrooms and the parson’s office and secretary's office were on the first floor.  A full commercial kitchen on the second floor.   This was quite a large building.

When the dead men moved the boiler that fed the three steam mains, the zone valves were included in the project and for the life of me, I don't understand it but there was no banging and the valves when closed did not create a problem with the one zone that was open.  And that happened a lot.  There are other stories about this building that I have shared on HeatingHelp.com but I figured this one about the zone valves was appropriate for this discussion.

I was a little nervous when the church wanted a replacement boiler.  I called Weil McLain and asked John McGarry (my WM Rep) to look it over with me before I bid the job.   I replaced the 2” & 3” screw threaded valves with welded flange valves and included new thermostats.  I also converted the condensate pump into a boiler feed pump.  The float in the reservoir would bring in fresh water if needed.  

So if you think that just adding 2 zone valves is gonna be enough, then you are going to have a bunch of problems.  You will need an expert with an intimate knowledge of steam, purchase two traps and a condensate return pump as a minimum, and maybe some additional piping designs.  After purchasing all that extra material you are probably more than half way to purchasing a second boiler.  And that second boiler will have less headaches than one bigger boiler with all the extra stuff to make your idea work.

I would stay away from the zone valve idea on your job unless it is absolutely necessary.  In the future, someone may want separate heat again and your redesign will make it very expensive to do.  If you are getting a gas boiler (or 2 gas boilers) then making separately paid heating systems in the future is just a matter of adding a separate second gas meter.. 

Just some more ramblings from an old man

That said. If I was installing two boilers, I would leave a tee in each boiler's condensate return and a tee in each boiler steam supply.

In an emergency if one boiler fails you can make a couple of temp connections with copper and heat both sides.

I did this in a pretty large Town hall once and tied 2 3" steam systems together with 1 1/2" and it worked well enough while one boiler was replaced. And that trick got me the job because they didn't have to go without heat.