And this part will also need to be adapted for more than two machines:

add $IP1 $IP2 $ENCAP 15700 $CRYPT "$PASSKEY";
add $IP2 $IP1 $ENCAP 15701 $CRYPT "$PASSKEY";

Those numbers (15700, 15701) are the "Security Parameter Index" values, and should probably be unique for each IP/IP combination/order. And I think they still have to match between the server's entries and the remote entries. Any script you use would have to take that into account as well.

Cheers