TOPIC: THIS IS MY PLAN....BUT WILL I STICK TO IT????

That if an effective formula for learning 2 instruments

Now I can only speak for my self but that is where the main difficulty comes in for me. My focus tends to have ebbs and flows, but when I'm focused I'm hyper-focused. I'm not the best multi-tasker in the world. Mainly I pick one and focus on it for a while and then go back to the other. I play fiddle and some viola, but right now mostly fiddle. I also try to do some mando, but again, quite intermittently. I would try the same approach as you would with friends: to maybe split up your time as evenly as you can (unless you're learning a new song or something). Do your best to keep 'em from getting lonely or jealous. That being said, if you've played one instrument longer than the other, then you might need to invest more time in the new one to get it "settled in," so to speak.

Also one thing that certainly helps me is to do some of the new songs I'm learning on one instrument and take them for a test drive on the other. That approach gives you an advantage because you're A. Actually practicing, B. Practicing the songs that you're working on, and C. Practicing that new instrument.