Hi Arrgh,
As Mirt and Storm already said they do not stack. On my T-hunter however I kept Imbuing as fights rarely need extra mana and I like to recall to Queen's Forge after each chest to unravel and than go home. Of course this is personal preference but the main point is that both Focus and Imbuing give same bonus to Mysticism so whichever one you keep will not change your damage output (if you have mana to sustain it)
Z