You would need to take a feed from both to get any benefit from using both, especially if they sounded different and the point of using two amps was to get the blend between the two.
If there is no PA support for the bass then I suggest to the OP that they get a long lead or a wireless system and go around the the venue and check just how much tonal variation there is in different parts. What you probably find is that one amp/cab combination projects far more efficiently than the other, and while the tonal balance between the two is perfect where you are stood on stage, it will be completely different out in the audience.