You would not stack them, because thp do not stack- You'd end up with whichever power gives you the most THP.
However:
If you were a Battlerager Vigor Fighter, with the dilletante power of Bolstering Strike, then the thp -would- stack, because Invigorating thp are allowed to stack with other thp if you're a Battlerager. (and yes that is mighty)