You definitely can't be a Cleric of two different deities anymore than you can be Fighter of two different archetypes...
A somewhat more interesting question is ... can you be a Paladin of one deity and a Cleric of another? I think the default position would be a pretty hard no. But at the same time, a 5E member of the Orthodoxy could make a very plausible story argument that they are working on something like Dual Faiths. That seems like an interesting idea. At present, TMK, it isn't forbidden...
Its outside the Mother Church and the Orthodoxy, but the Deathmongers/Divine Trinity also incorporate the faiths of multiple deities. Their rules are in the Blessed Lands book. Not sure if you've given them a good look or not. If you like Neroth, Beltine and Sarish (and really who doesn't?
) then its worth a look.
My 5E Character, Brother Czesslaw val'Holryn, is one of them.