The steam humidifier unit itself has to be like ~3 feet away from the chosen air handler, so no, it won't run steam to both air handlers, but depending on the size/layout of the house, you might be able to get away with just using 1 unit altogether.

I have a ~3500 sq ft house which has separate heat pumps/air handlers for upstairs/downstairs. I was able to get away with using just 1 Aprilaire steam humidifier for the whole house (downstairs). When setting the controller past 3/7 I was getting condensation on windows, etc. given how much steam it was pumping out, so it's definitely a very strong/capable unit when run @ 240V for that size structure.

In contrast, I also have a 10K sq ft house where I started with 1 unit (downstairs) and when cranked to 7/7 it does well for the basement/first floor, but wasn't reaching the top 2 levels. Added a second steam unit last winter on the upstairs air handler as well and now the whole house is covered 100%.

The neat thing about the controller on these is that it comes with an outdoor temp sensor that helps automatically adjust the level of moisture necessary inside given the exact ambient temp outside. This saves you from having to make constant adjustments to the unit itself as the outdoor temp changes, which keeps the unit from putting too much moisture in the air inside the house, etc. Simple to install as well; I utilized 2 unused wires that ran to the heat pump from the air handler and installed the temp sensor on the plastic fan shroud of the heat pump itself.