Yes. For one thing, Bear's Endurance does not provide extra hit points - it provides an enhancement bonus to Constitution. This does raise your hit points, but indirectly. Second, the hit points you gain from Bear's Endurance are regular, run of the mill ones, while Aid provides Temporary hit points.
Yes and no. Bear's endurance raises hit points as "side effect" of granting an enhancement bonus to Con. The relevant magical effect is an enhancement bonus to Con, not the granting of additional hit points. Aid grants temporary hit points. There's no overlap between the magical effects of the two spells. Hence, there is no question of "stacking" per se. It's like the bonus to attack rolls which results from increased Str from bull's strength combines with the +1 morale bonus to attack rolls from aid[/].