There are two branches in Teacup which attempt to support two extruders,
multi-extruder and
lumentino. Neither of both is mature enough for inclusion onto the experimental branch.
multi-extruder supports Gen3-style external extruder boards only,
lumentino is fixed to dual extruders (should support an arbitrary number of them).
You can use them as-is, code contributions for maturing them are always welcome, of course.
