You get +3dB from doubling the power. Starting with 1W, here is the sequence:
1W
2W
4W
8W
16W
32W
64W
128W
256W
512W
1024W
2048W
As you see, after you hit about 500W, it doesn't really make a lot of sense to keep doubling the power. FYI, I own and Ampeg SVT 8-Pro that is rated for 2,500W at 2 ohms, so it can be done.
The sensitivity rating of the cab is another factor. Say you have two cabs. One cab is rated 95dB 1W/m and the other is rated 103dB 1W/1m. This means if the first cab is fed 1Watt and the measurement is taken at 1 meter, you read 95dB. If the second cab is fed 1 Watt and the measurement is taken at 1 meter, you read 103dB.
In order for the first cab to play at 103dB, you need to feed it 8dB more power. So power must be increased by a factor of about ~6.31. No problem to go from 1W to 6W. But if you are starting at 400W, you need to increase the power to about 2,523.8W to get the desired 8dB...and this is probably not very reasonable. Chances are, either your amp won't have a enough juice or you speakers will not be able to handle it.
The point in the previous paragraph is to choose a cab with sufficient power handling and also the highest possible sensitivity rating. Then you don't need 2,500W to get loud.
The next thing to consider is what happens when you add a matching cab. Usually the connection to the amp will be in parallel. The impedance is cut in half, the power handling doubles, and the sensitivity goes up by about 3dB.
So let's assume your amp can safely push both cabs and provide 400W at 8 ohms and 800W at 4 ohms. Notice the amp provides 3dB more power at 4 ohms than 8 ohms.
Let assume we have two 8 ohm cabs with a 500W power rating and 95dB 1W/1m sensitivity.
Two cabs will present a 4 ohm load, handle 1000W and have a combined sensitivity rating of ~98dB 1W/1m. Notice two cabs have a ~3dB higher sensitivity rating than one cab.
With one cab at 400W the calculated SPL is ~121dB. Probably not even close to loud enough (IMHO).
Calculated SPL [Log(400) x 10] + 95 = ~121dB
With two cabs at 800W the calculated SPL is ~127dB. If this is not loud enough, find another band.
Calculated SPL [Log(800) x 10] + 98 = ~127dB
For fun, 800W into a cab with a 103dB 1W/1m sensitivity rating:
Calculated SPL = [Log(800) x 10] + 103 = ~132dB.