How to reformat OS Catalina external 2 drives into one?

Yes, if you erase then all data will be lost.

You have to select the whole drive above Mac HD & Mac HD Data, usually in Disk Utility's View menu>ShowAll devices.

https://support.apple.com/guide/disk-utility/partition-a-physical-disk-dskutl14027/mac

Bombich Software, makers of Carbon Copy Cloner, has a more detailed explanation on their blog. (And note that if you mount a Catalina drive in Mojave, both volumes mount correctly, but appear separately and unlinked.)

Catalina does that on it's own though generally it hides one, the first one is a Read Only Partition with the OS on it.

OK, stay well. :)

Oh boy, it's tough, I'd say overall I only hear back from about half the the people I reply to, 50% of the ones that do never award points, 10% award the points to themselves, & I'm unable to solve some & of course other helpers may solve it instead of me.

Back when it was easier to get points awarded AND the forum was much more useful, I scored about 0.089 points per post... so don't feel bad. :)

Ok, every working then?