Wonder Woman I presume is easy. Paradise Island just exists in one of the bodies of water near Europe and is veiled by magic. As her film takes place in WW1, that is easy. She stayed hidden during WW2, maybe joining SOE or org before SHIELD. Diana herself is a demigod/god like Thor (same level of power).
Aquaman is slightly tricker as Marvel will want to introduce Namor as the ruler of Atlantis. We could have DC's Atlantis as a different kingdom and give it the new name Atlan, after its first king, or make it Lumeria. This way we get to keep its canon relatively intact. DC Atlantis's history before sinking can be placed before Marvel's Atlantis or at some point to better fit it in with whatever Marvel decides to do with Namor.
So it can can be done.