Can a Tahm Kench swallow another Tahm Kench that swallowed something?

In your scenario red Tahm swallows red Jinx and gets swallowed by blue Tahm. Blue Tahm will then spit out red Tahm before Jinx leaves.

If you turn it around blue Tahm swallows red Jinx. Blue Tahm then gets swallowed by Red Tahm. in this case Red Jinx will leave before blue Tahm.

In the first case red Jinx will be able to leave before the max time (6s) runs out.

In this video you can see a "Tahm Kench-ception" As you can see the ones that got eaten first will leave, regardless of how deep they are in.

Remember that Tahm Kench can keep allies in his mouth longer than enemies (6s for allies, 3s for enemies). So you'll spit out enemies faster than you would allies.

The only way this happens in a game is either in a one for all game mode or in a blind-pick game where 2 Tahm Kench are present.

Here is a nice clip of both enemies and foes Tahms eating eachother.

Every Tahm will remain in a belly a given amount of time. So it doesn't matter if the Tahm that swallowed your Jinx is swallowed himself, Jinx willl get out anyways after a fixed amount of seconds (even if the Tahm that eated her is still in the belly of another Tahm - uncanny, isn't it ?).