One coat is really good looking on it's own and will last a good amount of time. I have however talked to people over the phone that have done 2 layers and they stated that they did indeed noticed an increase in the gloss! Just ensure to wait 12 hours between the 2 layers and also 12 hours after that 2nd layer!
Also saw in the last post you had about using the PROFI Booster over the SiO2 Paint Sealant, it will actually work great over the SiO2 Paint Sealant and will add a significant amount of added protection. The trick to using that product is to use it extremely sparingly as it's very concentrated (a little goes a long way!). What I like to do is mist a little onto a towel, wipe a 3ftx3ft section, then immediately buff it off with a clean microfiber towel. When done this way, it will be much easier than spraying a lot onto a surface and wiping as it's just too much product being applied. Once again, this product is extremely concentrated! You can even do a 2nd layer of that PROFI Booster after 24 hours and it will add even more protection. 2 Layers of that PROFI Booster JUST standalone is insane durability (well over 6 months easily!). So when combined also with the SiO2 Paint Sealant, I think it's an incredible combo! Hope this helps