Use the Aux out on the Studio 24 going into ch 4 on the 1024USB. Control the balance of instruments by varying the individual Aux level controls on the Studio 24 while keeping the channel faders at zero.

I'm concerned when you say the Beta 52As are "combined" on ch 3 of the 1024USB. Can you clarify what you actually are doing here?

Welcome to RO.

Having a stereo feed to in-ears, to give the guitars and vocals a little separation, is often a good thing. If you wanted to try that, you would use the 2x (what appear to be balanced) 1/4" Alesis Mon 1&2 outputs on the back (which have built-in panning capability), and run them into 1/4" input jacks on channel 5/6 , with the level button set at +4 on the Behrmixer.

That would also free up the Ch#4 XLR for your other Beta52a (double kick mics, I'm guessing), without the need for combining them.

You would need 2x balanced TRS 1/4" cables to try that. Normal TS 1/4" instrument cables would probably do the job, but TRS would be preferred.

EDIT:
A Word Of Caution: Start with the Alesis Mon Output levels at zero and the Behrmix channel low. Ease the Behrmix to Unity and then VERY slowly start turning up the Alesis Sends. No sense blowing your eardrums out all at once. In-ears can be great if you can use them to reduce overall volume, but if you go overboard with the volume they can cause permanent hearing loss in a hurry.