It all depends on your desired carry system.
The first thing is that it is always lighter to carry one fabric carcass instead of two. So, carrying two bags or bag/quilt is going to be heavier for the amount of down insulation required.
However, versatility might out-weigh this for some people. I use a two bag system, but I do it differently. I have a very light MontBell UL SS Down Hugger "Thermal Sheet"(13 oz) which is basically a hoodless bag good for 50*F that can open up fully like a quilt if you want. I use that for hot summers and part-way into the shoulder seasons until I see night temps below 50*F.
Then, I switch to my WM Alpinlite 20*F(1 lb, 15 oz) bag for anything between 20*F-50*F.
If I need to go out when it will be below 20*F, then I will take both. This way I only carry both in extreme cold situations.
Other people use clothing to push their bag limits down to lower temps. I don't bring the extra coat, and instead I use my sleeping bag as a wrap/cape to keep me warm at camp.
For me, I find that the 30*F bags are neither fish nor fowl, and I want warmer when it's cold, and I want lighter when it's hot.
So, I do what I do.
It is a very personal thing to arrange a sleeping system that suits you. It is very hard to recommend something, considering the individual nature of sleep comfort, carry preferences, and clothing use.