One toilet bubbles when another is flushed because the flush pushes air down the shared drain, and that air cannot get out the normal way. With an open vent and a clear line, the air escapes through the roof. With a partial clog or a blocked vent, it is forced back up through the other toilet’s water.
A flush sends a slug of water down the pipe, and that water pushes air ahead of it like a piston. The vent system exists to let that air in and out; the International Plumbing Code requires venting that keeps the pressure on any trap seal within 1 inch of water column. When the air has no clear path, the nearest trap gives way. A toilet bowl is a trap, so its water burps as the air pushes through it, then settles once the flushed water has passed.
The pattern points to the cause. If sinks, tubs and showers are slow too, and the lowest drains in the house act up first, suspect a partial blockage in the main line. Rule that out first, because a main line clog can end in a backup, as the guide to toilet bubbling causes explains. If every drain runs fast but the toilets still gurgle, suspect the vent; here is how to tell if a plumbing vent is clogged.
One honest limit: a single gurgle now and then can be harmless, and it does not prove anything is blocked. Bubbling on every flush, paired with slow drains, is different. That pattern means the line needs drain cleaning or a camera inspection to find what is in the way. Until then, go easy on the toilets, since a partial blockage can turn into sewage coming up through the lowest fixture in the house.