Hello @Aaron_Westberg,
Since this is currently not avaliable (and may never be given the complications of having such 2-way sync as described in other posts), have you considered building the relationship like this? Each table would have a direct select option to the other table, and you would use the “All Related” columns for all of your doc reporting, formulas, etc. when you need to access the entire many-to-many relationship.
I hope that helps, thanks!
Edit: Switched the lookup columns to allow for quick adds per @Jean_Pierre_Traets 's suggestion - makes a ton of sense in context of a use case like this!