This depends, up to a point, on your goals. In general, it's no bad thing to become 'flexible', and the body and brain quickly understand what's going on, and make the necessary adjustments. This is helpful further on down the line, when playing other guitars. If, on the other hand, the idea is to become virtuoso in as short a time as possible, then concentrating on one instrument will work slightly better. It becomes a case of 'painting oneself into a corner', but if the goal is to be in that corner, then why not..?
Learning guitar (and other instruments...) is a medium/long-term affair. Be patient; things will happen far more quickly if you go slowly. Play one, or the other, or both; the essential thing is to play.
I've several guitars and basses (disclaimer: I'm a drummer...), and go from one to the other with no qualms. OK, I'm rubbish on all of 'em, but that's down to me being rubbish, that's all.