I believe that GroupMove+2 from an Elephant should stack with the GroupMove+2 from a HorseLord, since for every other bonus it is [Max Unit] + [Max Hero] as well, so it would seem bit odd otherwise.
I actually believe that these do stack correctly, but the GOLD MEMBER MOVE PREDICTION FEATURE does not show them stacking for sure.
Steps to reproduce:
Take a stack with a L1 HorseLord, Elephant and Ram with the same number of movepoints leftover.
Then select the ram and some far away place to see the movement points for the subsequent turns. Adding the HorseLord or the Elephant increases the shown movepoints by +2, but then adding the other one afterwards does not increase the shown movepoints by +2 again, which it should.