If you are not confident that you can handle both reins, I might be tempted to use a proper cavesson and have the lines on the nose until you knew you were ok.
The other factor is what your horse is used to.
You would also want to know that your horse was fine with having lines round his quarters before you tried this. It is possible to have the outside line come over their bum - but I've found that with a saddle it can get stuck under it - meaning you don't get a proper release or contact. If you work with a proper roller this risk reduces but you still need to be confident with the lines.