You are talking about horizontal scaling. It's something which is non-trivial and would need the core of the app to be written in a very different way. Think of the challenges
- Your sub-instance and your friend's sub-instance which serves a common community can be geographically far apart
- They need to be synced
- Internal function calls which were once local now need to travel across the network
- Depending upon the heterogeneity of the hardware, there can be a large jitter (bad for UX)
The admin of my instance lemm.ee has tried to make horizontal scaling by decoupling some lemmy functions but pure horizontal scaling is really hard (glusterfs/spark).