Normally you can only use 1 psychic power per turn. Using the force wpn ability counts as using a power. So if my space marine librarian uses might of heroes ability then he cannot use the force wpn ability that turn.
I say normally as there are a few exceptions. Mephiston and a couple others with force wpns are allowed to use more than 1 psychic power per turn.
When opp. takes a wound from a force wpn. The psycker makes a ldrshp test. If he passes,then opp. is slain outright. There is a character/wpn out there that lets you make a check for each wound inflicted to see if you slay the opp. Cannot remember for the life of me who/what it is.
As always,reading the rulebook does help. Otherwise,you will always shaft yourself. See p.46 special close combat attks. Then look under force wpns. There is a nice big paragraph explaining the use of the almighty force wpn.