Is this combo overpowered, just a damn good idea to take, or nothing special?
It DOES take 2 Feats for either party, which is a pretty signifigant cost
To explain, as far as I can see, LL/LB 'kicks in' when you are on your last Negative HP before Death (meaning you stay 'disabled' with D-H, and the 1 damage from acting can also be 'transferred'), allowing you/the Eidolon quite alot of lee-way to act (as long as able to be 'fed' HPs by the other) though of course with the staggered condition (so, most benficial for the caster Summoner than the mostly-melee Eidolon, but still a nice deal either way).
Thoughts?
it can be a nice boon for you. In my oppinion you are often better of without Die Hard because you go straight from - here and fighting/being a threat to dead like a fish. with this combo that danger is a bit aleviated so it is nice to have. But not more of an must than to spend 2 EP and 1 feat (eidolons) at level 12 so that your Eidolon can cast True Strike 3/day as a swift action. :)
the only possible misuse i see is building the eidolon in such an way that it only is a bunch of extra HP for your summoner and keeping it out of the action. But in that case you are forgoeing the combat powress the eidolon might give you so i think even that is more than "fair game".