Not exactly your situation but I use two quilts;
* 6oz PL1, sized about right for me
* 2.5 oz Climashield XP, about 4 inches wider than the other.
I use them alone or together depending on temps I want to prepare for:
* 2.5oz alone ... not yet enough experience to know the limit
* 6oz PL1, I start to feel cool somewhere around 25*F
* the 2.5oz over the 6oz, I start to feel cool somewhere between -5*F and 0*F
edit: The tests of the two together were in a bivy. The others were not.
edit#2: I have one single data point where I was toasty warm at -13*F in a bivy on a foot or so of snow, using a ridgerest pad on top of a 1/2" bluefoam pad using a 30*F bag (Marmot Arroyo) topped by both quilts. That was definitely not a lower limit of that combo.