You can use this program called Project Controller 4. It has the ability to change the version just for multiplayer. Once you exit JJ2 the version will be reset. PC4 is an advanced server tool. It comes with an instruction manual in case you can't figure out how to use it.

Note: The animations may not be entirely correct so in multiplayer things may look a bit wierd. This is because 1.23 and 1.24 don't use the same version of anims.j2a (the file that stores the animations), meaning that they won't have all of the same animations(1.24 has all the animations from 1.23 plus more animations).