I haven't seen any official statements from GUE either encouraging or discouraging mixed CCR and open circuit teams, except that I think GUE instructors aren't allowed to use rebreathers while teaching classes where students are on open circuit. In practice I know mixed teams are common now and may be the only way for Tech 1 divers in some areas to get additional experience.If I were to dive in mixed teams with CCR, it would be easier, but I don't know if that is encouraged in GUE - or is that an advantage of the GUE-JJ to facilitate mixed teams?
It's probably best for one open circuit diver to join a team of two CCR divers because the open circuit diver likely won't be familiar with the CCR pre-dive protocol or failure management procedures. Like, if the open circuit diver sees their buddy's CCR O2 regulator leaking are they supposed to shut it off?