The verb mingle means to mix or bring together in combination. Because mingling involves two or more individuals and cannot be one-sided, the prefixes inter- (meaning between; among) and co- (meaning jointly) add nothing that is not already contained in the meaning of mingle. Hence neither co-mingle, commingle, nor intermingle has any reason to exist, yet all three appear surprisingly often.
Examples
In each of the following sentences, co-mingle, commingle, or intermingle could be replaced with the simpler mingle:
And Udall gave him the biggest insurance policy by leading the campaign for Republicans and Democrats to co-mingle in the audience. [Union Leader]
Opponents have derided that measure as “the bathroom bill,” asserting it would allow the sexes to commingle in public bathrooms. [Boston Globe]
Weeds can be a problem in new asparagus beds because well established roots will intermingle with the asparagus roots. [Maple Ridge News]
For the beer, you’re in luck if you live in a state where food and beer happily co-mingle in the grocery store. [Washington Times]
Rugby forced me to come out of my shell and to commingle with my classmates. [The Daily Campus]
