Two sentinels will be good until around lvl 25 then they'll be ok but then around 40 you will absolutely kick butt! Especially using the tank companion. Kira won't help as much b/c she is DPS just like the sentinel. I think the tank companions are really usefull for the DPS class.

You might have better success with a sage/ guardian team with DPS companions for a more well rounded team.

I know you were asking ppl to speak from experience, but you should know that I've only 4 boxed this game. I do have a 4x sentinel team though.