Simply merging the two feeds doesn’t make sense. But there are, I’d say, more than one level of integration, all of which are useful.
Single app that allows you to sign in to both platforms with two accounts. Both platforms are presented in separate views, but you only have one app that also manages all of the peripheral preferences like theme, notifications etc.
Single app that partially fuses the two platforms without simply providing two completely different views.
EG … all feeds of communities/magazines can be listed under “groups”, all users under “people”, and all notifications from both platforms (including “DMs”) under one list/feed
Some special aggregation service that you only need to create one account for but that provides you access to multiple platforms with proxy accounts. IF such a thing were to exist, there’d probably a whole range of possible hacks .