TRex supports testing complex typologies, with more than one DUT, using a feature called "client clustering". This feature allows specifying the distribution of clients TRex emulates.
I've found a good blog post that describe how to use this new feature:
see here
http://the.vpn.center/2017/01/up-and-rawring-with-trex-ciscos-open.html?m=1
documentation
https://trex-tgn.cisco.com/trex/doc/trex_manual.html#_client_clustering_configuration
thanks
Hanoh