While I am not fully immersed in Behringer, I would say technically you could probably stack up three midi-controllers, have a unit with a dual-output (master/cue) sound card in there and get it all working (with lots of headaches and mapping).
That said, the idea behind modular is not to hook up two all-in-one controllers to a midi-only “mixer”.
You’d have to have at least a separate 4-stereo channel external sound card in the setup as well.
Personally I would not want to go down this road you are thinking of.
Better start saving up and get a good 4-channel/4-deck controller (something like the Reloop Beatmix 4 mk2 around 300 euro is a good option).