[Feature Request] Add matrix-matrix double puppeting bridge.

[dreirund]

Is your feature request related to a problem? Please describe.

The reference Matrix client, "Element", does not support multiple user accounts, as do quite some other clients also not support.

This is a long time outstanding issue with Element.

Managing and using multiple Matrix accounts is a daunting task, though, and easy to forget about one account.

Describe the solution you'd like

Add a double puppeeting bridge that can bridge between two matrix accounts.

E.g. ↗ this project matrix-matrix-bridge.

Describe alternatives you've considered

• Using other Matrix clients
  → not suitable for me because of lack of good user experience with nested spaces.
• Starting different clients with different profiles → bad user experience because of separate "programmes"/ windows needed for each account, and manual hacks prone to misconfiguration and messing-up.

Regards!