There are few item floating around that has +concentrate, can i activate the skill then switch to my main weapon so i can get the concentrate buff?

Yes, i have on F3 slot 2x weaps with +3 concentrate and +3 venom armor. I buff myself with switching to them, be aware, that concentrate can be used only if u don't move after casting it.

technically you can but it's just not that practical.

It is, why not ? At all the skill is useful only in full parties with tank at any boss. Putting points on it is a waste, and is very easy to cast it with F3 switch, i made my weap switch with 1-3 keys, and with mouse 1 on n3 the venom armor, and mouse 2 conc dmg. So the cast takes still 1sec. And while Hellfire takes 2ec. cd when you switch weapons, arc legion, flameshards and demonspine doesn't have any, so u can swap weaps and cast very fast.