ok so a month ago I X-fered my Char to another server, I am a Frost-DK and had Frost Presence activated.
I had 28k HP (I think) before transferring and after the transfer I had 31k
and I wondered where it came from and thought that maybe I now have Frost Presence twice so I tried switching presences and sure as hell I had my 28k back...
So after a month was over I trasnferred back to my old realm, again in Frost Presence.
This time I had 30k before and 33k after (got some stam^^)
So I walked the land with 2 Frost Presences active and never switched. I died, I relogged, I BG'ed. Only thing I haven't tried is arena!
Nothing out of the ordinary just +20% HP, other bonuses didn't add twice.
And today, with patch 3.1 deployed on EU Servers I saw the incredible: I had 36k HP unbuffed! so I think I now have Frost Presence THRICE! +30% HP because there is NO WAY IN HELL I get that far with my **** gear which is like 2 naxx epics.
TL;DR Version:
1. Transfer your Char with Frost Presence active
2. Never switch Presences again
3. Rinse and Repeat as often as you like
4. ???
5. PROFIT!!
EDIT: Switching specs DOES NOT get rid of it! Enjoy!
EDIT2: Screenshot impossible to archieve this HP unbuffed with so crappy gear