Yeah, but he specifically said the 115 and the 215 were both 8 ohms. The speakers would definitely not be receiving the same amount of power.
Exactly. The 8 Ohm 115 cab has one 8 Ohm driver - simple. The 8 Ohm 215 either has two 4 Ohm drivers in series or two 16 Ohm drivers in parallel.
If they are using the typical method of connecting two cabs together (either at the amp w 2 outputs or chaining them cab-to-cab) they are 99% probably connecting them in parallel. In which case there are different currents being presented to the 115 driver and each of the two 4 Ohm or 16 Ohm drivers in the 215. That's just physics.
Also, if you read through the TS parameters of drivers there is usually a bit of difference between the different versions of even the same model driver (the 8 Ohm vs the 4 Ohm or 8 Ohm vs the 16 Ohm, etc.) which will impact their character as well.
So, pairing up "matched" cabs isn't just about matching their diameter or brand. It is about matching up the same driver in the same box with the same tuning (ports, batting, etc.).
Again, all of this about getting the best sound you can everywhere you can as best you can.
Most of the time I see guys playing mixed cabs etc. stacked whatever way either looks best to them or balances or fits their rack on top etc. made up of whatever cabs they can afford, find, borrow. And usually nobody in the room notices (except the bass players and the sound guys) and they still rock out and they still get paid.
I just saw a seasoned player have his Markbass 115 combo on top of a mark bass 210. Probably the only way he could combine them and get to the controls. It was loud and thumpy but probably could have been better.
But, if you are going to ask for recommendations... and you have the CHOICE and the MEANS to get what works best, then it kind of makes sense to consider these guiding principles don't you think??