Just an idea. Lets say we have an AP with two removable antennas. What if we take one antenna to the other side of the house with a low loss extension cable?
Should it work ok? It would be easy way to extend coverage without buying more access points, mesh, whatever.
In this case there are only a couple of users.
Feel free to critizise...

Decent RF cables have loss of approx 1dB/m ... but those are not exactly thin, they are around 6mm in diameter.

If AP has two antennae, they are intended to be used in 2x2MIMO configuration. In your use casd they would be used as 1x1SISO ... but that's not the way hardware is usually built. Which means it might work as intended, but more likely it will not. For example, it might be that beacons are only transmitted on single Tx path (chain) which means you wouldn't have wireless coverage over the other antenna. Or some other equally unpleasant effect would happen.

OK, thanks for these comments. I need to study more about this antenna idea. I don't have a cable to test it. But like I wrote, "just an idea"!
Cable itself should not be the problem. Just more Tx power to cover the cable loss. (Of course there is more attenuation on Rx side too).

Now, so far I understand, AP with 2 antennas is so called 2x2 AP. It have two Tx-Rx streams. If first client takes connection, it will get first stream from first antenna. Second client will get second Tx-Rx stream from second antenna. Both can communicate same time, they don't have to share stream. If so, taking one antenna away with extension cable should work fine?
Of course then one client can't have benefit to use both streams at the same time to increase speed. But if the connection from the ISP is only 100M, there is no point to do so. One stream is enough

If first client takes connection, it will get first stream from first antenna. Second client will get second Tx-Rx stream from second antenna.

No, MIMO allows single client to communicate with double (tripple, quad, ...) speeds. The way you explain is called MU-MIMO for "multi-user" MIMO and that's advanced. In typical cases AP (or 4G/5G base station) uses advanced antenna array which then intelligently adapts antenna pattern into barrow beams to hit client with higher signal strength.

What you're aiming to achieve is usually called "space diversity", but as I already explained not all wireless systems are designed to transmit beacons over all antennae (4G transmits reference symbols over all Tx antennae) and if beacons are only transmitted over single antenna, then clients won't detect wifi AP in vicinity of the other antenna.