I typically like a puffier batting with less quilting than is currently popular. A lot depends on how it will be finished.
I have certainly doubled thinner batts, but would hesitate doing thicker ones before I did some tests. Like, if I knew I was just going to tack down the corners of the blocks maybe using buttons, I'd go ahead and make it as thick as you want. If you and your machine can handle it, go for it! But again, before I was committed to the double batting I'd do some tests using the quilting design I want. You want to end up with puffy, not a mattress pad!