Most stereo EQ plug-ins will allow you to split the EQ so you can have different EQ settings on one side to the other.
These two screenshots show the Pro Tools EQ3 plug-in. I have de-selected the stereo link button and then set matching HF cut and boost settings on the left and right sides. However the EQ3 won’t display both sides simultaneously. The Waves Renn EQ is an example of an EQ plug-in that does show both sides simultaneously making the set up much easier.
This produces a very subtle width enhancement and providing you have matching opposites this trick will be completely mono compatible, as the 2 effects will cancel each other out. Alternatively you can duplicate the tracks and have an EQ plug-in on each track.