I did this recently on a group hang here locally. Woke up a bit cold the first night at around 5am, wife was even colder and decided to come home for the second night. I kept both her TQ & UQ (both from Underground quilts and both rated for 20*) and used them both.
I know the temps were in the minus (celcius), although I'm not sure of the exact temp for both nights, if I had to guess -4 or -5 plus a windchill. I deployed both sets of quilts for the night, I also added a hot water bottle between my legs, so I'm not exactly sure what gave me more heat, but I had to throw the top quilt off by midnight - I was roasting. I simply moved it over the RL and at my feet. I didn't really feel any difference from under me but it's really hard to say for sure.
In conclusion I felt like the doubling up the TQ's provided much more warmth than doubling up the UQ's. This is very subjective as I have no conclusive data to actually prove this. Also keep in mind that they are both UQ's are full length. I would try this experiment again given the opportunity but I also have a travel sock from Warbonnet on the way so I doubt that the chance this winter as I plan on testing the sock out. I'm really undecided what provided more warmth; the hot water or the extra TQ. I will say that I was starting to sweat with the second quilt on top before removing it.
Hopefully others have tried this and will chime in but certainly you can't go wrong experimenting yourself. As has already been mentioned make sure that you don't compress the one quilt with the other.