I develop multiple plugins and I want to load them together. So, I created a folder with minecraft_server1.8.9 and the last spongevanilla jar.
I put my mods in the mods folder. When I launch the server, sponge say there is only one plugin detected (always the same). If I delete thie plugin successfully loaded and let the other plugin it is detected without error.
So, it is not a problem with my second plugin.
Note that the same goes with any class - always keep 2 separate plugins in 2 different packages. Any overlapping classes could cause major mismatches when loaded together.