I finally have 2 upstairs toilets working after a lengthy bathroom remodel. What I noticed is when one toilet is flushed, the other toilet water level goes down, almost all the way. The lower level toilet is not affected.
Both toilets flush fine and there are no clogs. After flushing sometimes the water level floats up and down a bit.
From what I've gathered so far this is related to improper venting. I got on the roof and can't see any obstructions in the vent. I read about a technique for flushing by using a garden hose, but snow is preventing trying that now. Also I in the basement it looks like I can unscrew a cap on the pvc pipe to see if there's a clog.
This past year my basement flooded about 6", so perhaps that is related. Also had a new roof installed, so not sure if they dropped anything down the vent.