With the OP MIA, it seems the topic has changed.
Too often sound absorption is very much misunderstood. Much like a sponge with water, absorption is finite. Take a large sponge and point a water hose at it, that sponge is only going to absorb so much, and the rest is going to splash off. Years ago we mainly looked at the NRC and STC ratings when building sound treatment. Today the best way to choose absorptive materials is using Gas Flow Resistivity ratings. Like my sponge analogy, sound hitting insulation materials will penetrate until the resistance based on the materials ability to allow sound waves to penetrate, causes the sound to then reflect back. Simply put, at some point, sound absorptive materials all become reflective. The old adage, the best sound absorber is an open window because sound waves exit and don't reflect back.
GFR numbers can be used to model and graph across the audio spectrum and there are online calculators. There are a number of threads on other forums with common insulation products from around the globe with the manufacturers testing results with these numbers. Worse case, you can call a manufacturer up and ask, though some products they don't do this test.
When I recently redid my sound treatment, I modelled various available products and selected what would work based on the problem frequencies I identified via room acoustic measurements. Here is the practical problem with mixing and matching. You just don't know where you'll reach reflectivity. The only way to know would be to lab test and that is not practical for DIY. In all you'll likely add enough resistance with one material as to greatly reduce the effectiveness of the other.
OC 703 has long been the gold standard for sound treatment and is still currently being used in many commercial products. A big part of the reason for this is aesthetics, (i.e. it looks good). Problem is, it has its own issues. OC retested and the new ratings are not nearly as good. Maybe manufacturing methods changed but it no longer is as effective at low frequencies. Also add GFR numbers and model, you'll find that after 4 inches, it becomes reflective at lower frequencies. There became this mythology around it that survives even though so much of what we know has changed. In the old days we always assumed that if you added more, the lower frequency NRC numbers would add up to more absorption. It doesn't. In many of these products including 703, just going thicker isn't better at low frequencies. Generally, the higher the density, (higher GFR) the higher the resistivity. Low density pink fluffy stuff is more effective when going thicker for lower frequencies.
Point is, measure acoustically and engineer a solution that meets the needs. Or just throw stuff up on the walls and hope it does something.