If you want to combine the objects from two Object Tracks together, select the objects from one Object Track. Next, go into the Track Commands menu, and select "Move Selected Objects to Track". Then select the Object Track you want to move into and your objects will be placed into the selected Object Track.
If you want to combine two animations together, you can go to one Object Track, and save the animation as a Object Animation preset. Then, go to the other Object Track, and under Object Animations, select "Append Preset".