So you have a 400kb line, shared between two people and potentially more 'devices'.
It will suck, no matter what you do.
Presumably, both of you pay for the connection in equal amounts.
1. Contact your provider and get more line speed (and pay the extra)
2. Discover the joys of living with a roommate, and work it out. This is exactly the same as if one person was using up all the hot water every day, or always hogging the good chair.
Work it out.