Patriarch Svyatoslav, when he was in Rome for the glorifications, did mention in his interview that both pope-saints were closely linked to the UGCC.
There is certainly nothing barring individual parishes and eparchies from celebrating their feastdays.
I know of UGCC parishes with icons of the "Pillars of Orthodoxy" including St Mark of Ephesus.
I'm almost certain they didn't ask their Eparch for permission for that . . .
When I meet His Beatitude the Patriarch on Sunday, I will put the question to him!