W_D is correct--two separate bags will be quite a bit heavier than one warmer bag. Of course, using the two bags you already have will be lighter on your bank account. Only you can decide which is more important!

If, as you say, one bag will fit over the other without compressing the insulation, the combination may work. However, we all differ as to sleep styles and our individual body "thermostats." In other words, you'll need to test the combination yourself. I'd do the testing in the back yard (if cold enough) or car-camping before going out on a longer backpack. Be sure you have a nice warm pad (not a NeoAir!) underneath!