I just started using Summit, coming from Liftoff that seems abandoned and does not support Lemmy 0.19.
It seems Summit don’t have the ability to show all posts from all multiple accounts in single view, either Subscribed or All. Something for the devs to consider.
Actually I just went and read Liftoff’s source code. The “multi-instance” fetch logic is here: https://github.com/liftoff-app/liftoff/blob/d7fdeb0f948f0bb17c54617eb8c5b74f556fb01a/lib/pages/home_tab.dart#L377
The tl;dr is that Liftoff uses a very primitive way of handling it. From what I can tell Liftoff does not do any dedupping across instances so the same post may be shown multiple times. This actually makes the implementation much easier.