For the IPSec phase2 network selector, you can use 192.168.50.0/23<->192.168.0.0/24 to let them communicate each other. If you have tunnel interface IP and want to be able to test connectivity from either FGT, you need to add those selectors as well.
For the rest, you can use each subnet separately for routing, and have to have separate policies per interface unless you use a zone to bind internal1 and 2 together like "internal_zone".
Actually you can use the /23 subnet for routing as well on the other side of the tunnel.
Or, for simplicity, you can use a wildcard address '0.0.0.0/0' in phase2, for both local and remote end - if the other side uses a Fortigate as well. This will allow any subnet to open the tunnel. You take care of security by creating the appropriate policies.
Wildcard address comes in handy when you have to deal with a high number of remote subnets, or when you do not know the remote subnet address ranges beforehand. That is, in case of a dial-in VPN.
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