Yes. Whoever is upstream would need to flush first and then as that poop would arrive under the downstream toilet if that person flushed - whoosh - they could collide in the main line. But gravity being what it is… there is little chance of a clog in the downstream toilet unless further downstream is clogged and then it could back up all the way back to both of you! But that’s what the siphon is for so it’s not actually going to come up out of your toilet… (Usually)

This is why in hotels you’re going to notice that all the toilets and the showers and the sinks form parallel runs all the way along the building. They’re all going to go down into a horizontal submain sewage tube (4"+) which is going to be pitched from one end of the building to the other. On the bottom (far) side of that is going to connect to the vertical main sewage line that’s going to go down vertically below the ground to the actual municipal sewage system as well as vent vertically upward out the roof.