What is the maximum transmission power of a 4G/5G phone with multiple antennas?
It depends on how you feed the antennas and how your transmitter is made.

If you have a single TX feeding two antennas, this can be done only with a power combiner, so sending 50% of the power to each antenna (and loss in the combiner!)

Instead, with a specific TX for each antenna, what you say can happen: you can feed as much 23 dBm &quot;streams&quot; as TX outputs.
I have recently worked with a train-wayside application using 4 antennas fed by  4-output provided by a router that is a 4G/LTE/WiFi router (https://www.teldat.com/telecommunications/transport-routers/telecommunicationstransport-routersteldat-h2-rail-railway-train-metro-router-lte-4g-3g-wifi/).
I put a screenshot below: the first four N connectors are the RF TX output to feed 4 antennas, independently.