All you devices will try to send power up to the lnb when they are turned on and in use. Put simply it looks like your current splitter only allows power to pass via one of it's ports and its blocking the power on the other/s.
Typically set top boxes do not power the LNB when they are in standby which is fair enough. PC cards will fall into either always powering or only when directly in use.
If you reverse the connections, you'll have the opposite effect. i.e. you'll have no problems but your flatmate will only be able to use his box if your PC is turned on.
This may be ok or a problem for you. $10 will sort.
Openmedia: How would you protect the lnb from multiple devices powering it?