I generally try and get similar temp quilts, but I don't have it as a set rule. I agree that having the underquilt rated lower is better if you don't match.
Generally, most manufacturers (at least of hammock quilts) tend to produce things all in the same range - ie. 0* quilts, 20* quilts, 40* quilts, making it easy and simple to match. As the temp ratings are 20 apart, I probably wouldn't pair a 40* TQ with a 20* UQ in most situations. I'd just go with 20 for both.
As a note, Big Agnes tends to promise a bit more than they can deliver, temperature wise (although I do love the design). Your 15* Lost Ranger probably won't bring you as low as 20* Hammock Burrow TQ. So, depending on your specific needs, I'd go with something like a 20* HammockGear Incubator to match with the Lost Ranger. Depending on how hot you sleep, that would probably bring you down to the 20-25* range or thereabouts. And, if you have the money, throw it at a 20* Burrow TQ, GoLite 3 season TQ, etc. Not having that hood and extra material flopping around, plus the extra insulation those bring, is very nice.
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