That would indeed be a wonderful world where something like this would exist. However, I fear it's totally unrealistic.
As everyone can see, we're struggling mightily even with
single-platform syncing. It's year 5 of Marvin, and we still have no device-to-device automatic syncing of annotations. I know people who ditched Marvin over this, saying: "If it didn't get done in 5 years, it's
never going to get done." Well, I'm not that pessimistic.
PS: A few years ago, Kris quixotically approached the developer of Moon+ Reader Pro (clearly the finest Android e-reader), suggesting to find a way to enable at least the syncing of
reading location between Marvin and Moon. Nothing came out of that praiseworthy initiative, unfortunately – and not through Kris's fault, as I remember.