Geeking out for a moment -
 I have a concept in my brain for a manual transmission gear set to share common drive between each [cold side] compressor wheel of a turbine engine. Triple (triangular) counter shafts ⎊ splash-oil gears.

Theoretically, 1st compressor is also flywheel & clutch combo (or internal), and exhaust is scavenged around final drive output at turbine - to recirculate air to intercooler then intake for consistent boost pressure. Efficient, compact, closed, constant mesh. 🏎️