I am finding that when I bring two to the range I don't do as well.
Are they the same design? Operation? Caliber? Weight? Ergonomic features? Do you 'favor' one over the other?
Don't claim to have the definitive 'answer' for anyone except myself ...
I'd think it would generally be influenced by the knowledge, experience, training, familiarity with the handguns involved and the frequency of practice ... just to list some possible influences. A 'baseline' of familiarity and skill development ... and continued maintenance of same ... is often necessary for folks to develop and maintain safe skill levels with any given firearm. Sometimes mixing them may not be a good investment of someone's time and energy ... at least not right away.
Safety is paramount. Don't rush things. Stay safe.
Becoming frustrated with your abilities isn't the best of potential desired results, though.
Maybe you should pick one and invest some additional effort?
Any local ranges have some training classes which might interest you? How about some local IDPA events? Something to enjoy, but which would also help provide you with a safe venue in which to develop and hone your basic skills to a satisfactory level.
Myself?
I've been shooting handguns since I was 5 years old. That was 50 years ago.
I've been a LE firearms instructor for 18 years, and an armorer for a bit less than that.
None of that time was a cake-walk, however, or allowed for 'coasting'. Not once I decided I wanted to achieve 'more' in the way of consistent skills and abilities, anyway.
I invested some time and effort. The more I thought I knew and had achieved, the more I realized I had to work even harder. It's a self-satsifaction thing, for me ... as well as a work-related issue. (Nobody really has confidence in an instructor who can't consistently demonstrate what they're trying to teach, and without 'preparation' or 'warm up' time, do they?)
Long ago I decided the most important person I needed to avoid 'fooling' in the skills and abilities area was ... myself.
I often bring anywhere from 1-7 handguns to a given range session ... more usually 1-3 (unless I have to squeeze in a scheduled qualification with more personally-owned guns from time to time).
I find it doesn't really matter which gun(s) I bring, or how many different designs, calibers or sizes I bring ... I experience the same inherent comfort and results. Time, frequent practice and experience can do that for some folks, though.
For those occasional days when I may want to simply 'enjoy' a range session I may bring a single action revolver or .22 pistol. I expect to do well with either without having to 'warm up', though. The time invested to maintain skills and abilities ought to have some benefit, right?
I have a purpose for all of the trigger time I invest in when it comes to 'skills-maintenance' sessions with issued & personally-owned weapons I carry, after all.
Also, while there are those days when I may have the time and inclination to shoot anywhere from 100, 200, 300 or even up to 500+ rounds, I'm no longer interested in burning up ammunition for the simple purpose of doing so. Every round I fire has a purpose. Every round. I've had times when I received just as much satisfaction and benefit from firing 1 magazine load in a single pistol. "Quality" of trigger time versus "quantity" is my preference. I'll grant that this realization didn't occur to me until later in my personal development and experience than I would've wished, though.
I also try to add some of the more common pistols which we may have in our training inventory, or which some of our folks may have personally purchased for off-duty usage, but which either aren't issued or of which I don't personally own an example.
For my own desire I want to be able to demonstrate more than 'minimally adequate' skills with any handgun I may pick up ... and that takes time, experience and training to develop and maintain.
Shooting is a considered a perishable skill in LE, after all (as well as in many sporting, competitive venues).
I'm sometimes a bit dismayed by the number of LE folks who choose not to practice on their own time and are satisfied with being able to just demonstrate 'adequate' skill levels during qualifications, even if it requires some remediation during a given qualification session ... as well as CCW licensees who readily claim not to have fired their CCW weapons since their last renewal & qualification.
It's an individual's choice ...
Just my thoughts.
First and foremost always train & practice SAFELY.
If possible, seek out some helpful advice from accomplished range staff and trainers. There's always something worthwhile to learn ... and relearn ... and then remember again.