You can use both if you want.
Drag any Sonitus (or other) plugin you want into the Pro Channel and it will create an Fx Chain for you.
Interesting, but wouldn't the multiple compressors "cancel each other out"? Why can't you just take the average signal of the two compressors and use that in one compressor?
Not really. You can use this arrangement to accomplish 2 different tasks such as one to capture & tame excessive peaks and the other to achieve levelling, impart tonal colour, modify the apparent "envelope" of a signal, enhance loudness.
Put it this way, you wouldn't put 2 compressors on with the same settings, you would just enhance their effect.
If you go down this road, remember the the action of your ratio control acts as a product of the 2, not the sum
So if comp 1 has a ratio of 2:1 and the other 4:1, the combined effect is 8:1