I would just do
out1 = foo1(in1, in2);
out2 = foo2(in1,in2);
BTW there are no user defined functions atm. (EDIT except curves) It isnt high on the priority list since I am trying to keep the language small !
Unless you could support macros of some sort.
Not for a while 
. I plan to do conditional compilation
https://github.com/kwikius/mixer_lang/blob/master/grammar/bison.grammar#L26
but Expr must be a constant, no runtime at least for now
Simple Gui for the language.
The mixer implementation has been updated, so you can now embed some simple mixers in your FC using strings
as wella s files
https://github.com/kwikius/mixer_lang/blob/master/examples/mixer_from_cstring/main.cpp#L103