I've designed a dual extruder that has only a single hot-end. This eliminates the problem of getting the offset set correctly. However since both filaments go into the same hot end, the tool change g-code needs to retract the old filament to make room for the new filament. The problem I'm having is that on the first layer, on the first tool change, the seconday extruder is not getting primed into place.
Each extruder is retracted 45 mm when not active. On the first layer, initially the primary extruder is active, so the secondary is retracted to -45mm. When the tool change happens, instead of issuing a G1 E45, S3D is zeroing out the secondary extruder with a G92 E0. Consequently, the secondary extruder, even after printing the priming isn't primed. Furthermore if the secondary extruder isn't used enough, the subsequent tool change retraction will back the filament entirely out of the extruder.
I'm looking at the tool change script, but I can't seem to figure out how to make one that does what I need. I'd appreciate any helpful suggestions.