I don’t think that will work. At best, it would split the signal power and neither antenna would perform well. That’s not even getting into the RF wizardry which I’m not equipped to fully explain (or understand lol).

The “proper” solution would be to have two Meshtastic units. One in a high place for long range and configured with the “client base” role and the other one(s) with regular antennas optionally in the “client mute” role. The reason for that is they’d likely be able to “hear” the base station one just fine and relay through it and wouldn’t necessarily need to rebroadcast locally. If they do, then they can be in regular “client” role.

Since I have multiple units inside, I keep mine in “client mute” when I’m home so they go through my “attic node” and switch to “client” when I’m out and about. The 2.7 firmware makes this easy to switch directly on the device without having to mess with the app settings.

https://meshtastic.org/blog/demystifying-router-late/

Now if you’re talking a highly directional Yagi for the high gain/high place antenna, then I believe the general guidance still stands. The newer firmware has some zero hop configuration where you can “pair” devices (via favoriting) and while they still hop the signal, it doesn’t decrement the hop count.

https://meshtastic.org/blog/zero-cost-hops-favorite-routers/