Well, it is worth noting that if you're stacking the benefits of the paladin's mount and the druid's animal companion onto the same creature, it's going to be pretty strong -- even stronger than if you were using the Devoted Tracker feat to get a companion mount. Remember that a companion mount typically entails splitting your class progression, so while the creature is getting benefits from both the animal companion and the paladin mount features, you're not getting the full benefits from either one of these features singly that you could as a single-classed character. The net result can of course still be better, but this is a fair trade for the other features you're losing out on for splitting your class progression.
Now, personally I don't imagine this will actually be that much of a problem, since while the creature would be stronger than either a druid companion or a paladin mount alone, assuming the paladin actually uses the creature as a mount this sort of setup would essentially (as far as character resources go) amount to depriving the druid of his animal companion to give the paladin a better mount. That sounds like a good move to me, since druids are OP even without their companions and the paladin could use a leg up (especially where their mounts are concerned). But it's worth bearing in mind.
As far as rules and mechanical logistics go, you run into a bit of a problem in that the paladin's mount is actually a summoned creature that only sticks around for a limited amount of time each day, and furthermore is treated as a magical beast for anything depending on its type (though it still has the HD, BAB, etc. of a creature of the animal type). Thus, by strict reading of RAW, a paladin's mount can't be a druid's animal companion (unless, of course, you get some kind of feat or class feature --or DM ruling -- that says otherwise).
As for two people riding the same creature, I'd say going by the way the rules regarding riding are structured it'd be more a matter of size than strength. A mount needs to be one size category larger than the creature wanting to ride it, and effects that care about sizes of creatures or objects tend to treat creatures as being equivalent to two creatures of the next size smaller. So I'd say that, although I'm not sure if this is true by RAW, it should be perfectly reasonable to let a Huge-sized creature serve as a mount for two Medium creatures at once.