Wireless Joint

One modem in your wireless setup is not enough.
So you add another modem with load balancing.

You have amassed a 2x2 or 4x4 MiMo antenna array pointing at your favorite tower.
Assuming all cables are equal lengths and antenna are identical SISO's or 2x2 MiMo's.
Since a single modem UpLink uses either one or two antenna...

Can you share the antenna array with 2 or 4 combiners, making sure the two modem's Tx signals are using separate antenna within the array?

That would be one heck of a combiner.

Tuning would be a nightmare I would think not to mention the size of the notches width wise

Really?

I was not talking about anything bespoke with frequency management.
Just (4) 3-way splitters combining both sets of the (4) antenna Leeds of the modems/routers with the (4) antennas, the only issue, IMHO, was to prevent the two modems transmitting on top of each other.
The only issue I am now thinking about is having two independent transmissions too close to each other, that would create some interference, but that would be true regardless of combining the two sets of antenna.

Forget it! This would never work.
The more you know, the less you know. Was trying to have my cake and it eat to.
No way to share the antenna array unless you re-invent all the RF signal processing that the 4x4 module is struggling with in the first place.