So R values are - for all intents and purposes - additive. Thank you.
The plan is to put two Ridgerests under the Thermarest, to protect it from puncture and to make a thicker, warmer sleeping pad. I'm a rolly-polly side sleeper for whom a good night's sleep is worth the extra weight.
It also allows me to use a shorter (20 x 47 x 1 inch) and lighter weight (11 oz) Thermarest Prolite to provide shoulder to knee comfort while the two 20 x 5/8 inch Ridgerest pads (one 48 and another 24 inches, total 14 oz) form the bottom 72 inch layer, provide both cushioning and insulation for my head, lower legs, and feet. Total thickness = 1-5/8 inches; total R = 4.8; total weight = 25 oz.
I can get the same overall system weight and thickness by placing a 20 x 72 x 1.5 inch) Thermarest Prolite Plus (24 oz) atop a 19.5 X 59 x 1/8 inch Gossamer Gear Nightlite pad (2 oz), but the total R value (about 4) and puncture protection is much less this way.
Either system provides greater flexibility than just a Thermarest alone. I get a 48" backpad for my pack, a 24 " sit pad, and a sleeping pad, all with the same stuff. Also, if I should puncture the Thermarest and not be able to find/fix it, I still have an insulated sleeping pad, just not as thick and cushy as it once was.
The same system would work with the XS (36 inch) Thermarest Prolite, or any other torso-size inflatable (like the BPL pad), with a corresponding reduction in total weight.