Are you really using more than one external IP? In other words, what would you need the two external IPs for that one wouldn't do? A simple RDT rule does not justify it, it is a simple port forward and can be configured on different ports for different clients too.
If you still want to map external IPs to internal ones, you probably need a router/firmware capable of 1-to-1 NAT, or multi-NAT mode.
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