I understand your frustration, but my suspicion is that when the stat see more than, say, 1/2 to 1 degree below set-point it wants to call the 2nd stage to help.
That being said, it will run along just fine in 1st stage once it gets to satisfy the stat the first time. Don't panic as the "cold start" is tricky...it will level out when room temp catches up to set-point.
Only if you again ever bump the stat up would it bring on the 2nd stage...but not for long...if your H/X coil is sized big enough it will quickly drop back to only 1st stage.
As mentioned, a strap on aquastat could cure this...but how much propane are we talking about here? A little exercise for the propane keeps it ready for when it may really
A switch would work too but, if inadvertently left off your back-up will never work.
Here is a test to my claim above...lift/remove and tape temporarily the W2 wire at the furnace and let the boiler run up and satisfy the stat.
Without changing the stat set-point, re-connect W2 and see if on the next call for heat it is all
on the boiler.
Let us know
And there is always this option:
Now if you want to flip-flop, make FA Furnace Heat stage1...and boiler Heat stage 2, so if your away for a few days the FA Furnace will truck along on 1st stage only. When you would return home, build another coal fire and just switch off the FA Furnace "W Heat relay" (stage 1) and the boiler will take over on 2nd stage.
Call it a Vacation Switch