Multisite is not an option in v.4, it is always on by default and auto detected and auto configured.
If both sides are in bridge mode, with default settings, lan0 and wan0 are mapped correctly so that the wan0 points to each other then once TCP traffic starts to flow in both ways the peers will detect each other automatically.
The minimum requirement for bridge mode peering:
Default settings (e.g. Bridge mode both sides)
lan0 and wan0 cabled correctly in-path
1x TCP session (not bypassed e.g. HTTPS)
Firewall should not strip TCP Option 76
Optional, but most highly recommended:
IP Address