Logic Pro X 10.3 has a new plug-in routing architecture that allows the user to use any plug-in as either a joint stereo, dual mono, or mid-side processor. To activate the mid-side function, instantiate a dual-mono plug-in, click the configuration button, and set it to mid-side in the configuration interface. Notice also the configuration window allows you to solo channels. To exit the configuration interface, click any channel button and it pops back to the normal interface. Otherwise, simply close the plug-in window and it will go back to its normal interface next time you open it.
Note also, the left/right or mide-side channels can be automated independently of one another.
The only plug-ins that won't work this way are plug-ins that only work in stereo mode, like Valhalla UberMod, for example.
One last note, saving your plug-in presets using the save/load algorithm built into a plug-in inteface usually allows you to load it whether in mono or stereo. This helps because some plug-ins don't work that way if you only save it as a Logic preset.
I hope some of you find this information useful. I happen to really, really like this new feature...