This is usually the case if you are a person that has a lot of new animals coming through their facility. If you had a snake in the QT room on day 35 and you put a new snake into your QT room they would both start back on day 0. However there are still risks associated with QTing this way.
The way I look at it is you risk loosing whatever animals you expose the new snake to. Even with a separate QT room you still risk the chance of loosing all your QT animals if you bring in something that has lets say IBD for example. The only way to really be 100% sure would be to have one room for QT with one snake at a time in it for 90 days. This isn't always possible but when I can manage it this way I do.