If you are using the Tor Browser Bundle, your bittorrent client will not use Tor, unless you have configured it to do so. In this case, I do not believe that running the bittorrent client while using the Tor Browser is a problem. If anything, the bittorrent client generates non-Tor traffic that might help hide Tor use, especially if you are using obfuscated bridges.
It is difficult to configure bittorrent clients to properly use Tor. If you have tried that, and the client is performing well, much of its traffic is probably not being routed through Tor. Given Tor's high latency and low bandwidth, the client will not perform very well if you do manage to configure it properly. And in the process, it will max out many circuits, hog relays, and interfere with other Tor users. That is why people advise against doing it.