Hi James, if you have the spare jumper cable sockets you could try it and it probably will work.
The challenge is the Mayfly doesn’t have a lot of capacitive buffering and the LiIon battery performs that function. It might be more reliable to have a battery on each Mayfly and run the solar panel to both. Of course the boards need to be kept physically isolated so they don’t connect electrically.
My node is using a LT500/SDI-12 and a Keller Acculevel/Modbus on Mayfly 0.5B https://monitormywatershed.org/sites/TUCA-Na13/
Its using a release based on 0.28.5 ~ https://github.com/neilh10/ModularSensors/releases/tag/v0.28.5.release1_210711 and it is built as a working release in https://github.com/neilh10/ModularSensors/tree/v0.28.5.release1_210711/examples/tu_xx01 with src/ms_cfg.h_LT5KA_lte copied to ms_cfg.h