If you do it in the way other objects are combined (by keeping them as separate objects, but with same name and numerical value added after dot in the name) both objects should end up combined yet keep the texture.
So for example if one hair type is named "hair", you should rename other hair type to "hair.1"
At least thats what works with equipment, scene props and other object types, never tested that with hair.
Update: and now we know this doesn't work, since hair color is set via hair color slider ingame, this will work on only one mesh and the other will remain white