It should work. When you have the new electric tank installed, you may want to also plumb in bypass valves so that one or the other could be removed without losing the use of hot water at all. You may find that if you have a lot of company over, you may want to turn the propane one back on. Plumb them in series, with the propane one first in line from the supply. Having the water run through it would keep the water from becoming stagnant, and in the summer, would temper the ground water up to the garage temp and lessen the load on it. Might make it worse in the winter, though. Running them both would provide lots more hot water, should your needs be there occasionally.