Routing USB3 signals
For anything that fast I would be starting with a 4 layer stackup with buried ground plane and give some thought to the stackup. 

You can do 45 ohm lines (90 ohm differential) on 1.6mm DS FR4 but they are HUGE and not suitable for this. Your other option if 2 layer is important might be to go to a very much thinner board, 0.8mm is standard and thinner is available, but 4 layer does not have much of a premium over two and makes for much better SI.  

CPW may be a possibility but that has issues of its own if you cannot stitch heavily.