Welcome to Just Answer. I will do my best to fully answer your question(s).
Most likely you just have a partial blockage in the toilet itself. Give it a good plunger workout and see it that takes care of it. If not you should purchase a toilet auger from a home improvement store and rod out (snake) the toilet. If you start to notice other drains not working well then the clog is further down into the drain pipe system and will need the work of a drain cleaning company to fully remove blockage.
So if one toilet was working fine and the other had this problem and eventually both toilets are haveing this problem now this isn't a gravity air problem. So what pulls the water down the drain?
A slow or non-moving drain is almost always caused by a blockage either in the trapway of the fixture or the drainline downstream. Yes the vent can become clogged but that is more rare and would be investigated only after the drainline and trapway are confirmed to be free of obstruction 1st. Cart before the horse scenario.
As for what pulls the water down the drain, well air flow helps that with gravity and pitch but it has to ghave somewhere to go 1st. If piping is plugged all the gravity in the world isnt gonna push it through.
So if this has been going on for a few weeks and if there is a blockage/partial blockage would things eventually backup?