A Quick Question for you.
Do you think two audio 2 sound cards as aggregate on a mbp will be reliable for 4 deck traktor use?
Not 100% sure if the OS will see, or if the Driver allow a pair of Audio 2 on the same system.
I’m 99.99% sure that an aggregate device can be created and used in traktor my problem is whether it is stable enough or not.
If you can manage to get them both recognised by traktor as an aggregate then you shouldn’t have much problem with processing power. Especially with a newish mcb! you could always sell it on ebay and get a audio 4 
The audio 4 dj will provide 2 more stereo inputs but I can easily run 4 mono channels what do you think?
try it out OP, let us know what happens so that if anyone else asks we know what to tell them