Is it possible to use the rotobrush tool on one layer, while another layer remains visible under it?

I apply garbage masks to layers I want to rotobrush all the time. The idea is to have Rotobrush only work on the parts of the image that need the tool. I've seen dozens of projects where the users were trying to include something that could easily be rotoscoped in a couple of minutes with two or three keyframes. The trick is to do the hand roto first, pre-compose, then run Rotobrush on the Pre-comp.  It will save you a bunch of time,  and you'll get a lot better job. When Rotobrush is done, use that pre-comp as a track matte so you have a lot of control. If your roto work took a lot of time and it's very complicated I strongly suggest you Pre-compose the Roto work and render a DI (digital intermediate using the lossless with alpha preset) and replace the roto brushed comp with the file. There is an option in the Render Cue to do that and it could save you hours and hours of work fixing something that got broken by a random mouse click or an undo you didn't catch. 

do I understand well that you want to see another layer during the work with rotobrush? No, because the moment you're in the layer and don't have anything under it. You can use an Overlay view mode (red button with blue human under the viewport) and adjust it as you want to work comfortable.

but when you return to COmposition window, you'll se the edited layer and, under it, the second layer without any changes. Rotobrush works with layer, not with original of its content.