If I'm connected to the Internet with dialup and broadband at the same time, why is it only as fast as dialup?

I am using my cell phone as a dial up modem. As I move around my house, my laptop loses reception from my wireless router, so I connect to the internet with my cell phone as a modem. However, when both are connected, I have to disconnect my dial up connection because when both wi-fi and dial up connected, the internet is only as fast as the dial up.