They synchronize fairly well. Surely this has been done by someone who is both clever and creative. I keep thinking there must be some awesome way to incorporate a Diplomacy game into Space: 1889, perhaps with players running PCs in Space: 1889 and countries in Diplomacy to effect changes in political situations between the Great Powers on Earth as well as the other Inner Planets. Although I suppose the Diplomacy turns would be infrequent as 2 make a year go by.
Why specifically Diplomacy? There are other multiplayer games.
I suppose you might create a solar system map, or planetary maps of Mars/Venus to use in conjunction with Pax Brittanica. Would be kind of overkill, but the general focus/approach of that game (which is more about colonialism than direct confrontation between Great Powers) seems better suited.
It's about the same era, features the same national powers, and I own it.
I don't know what Pax Britannica is.