So you want it all to happen via one network interface?
What's the source device, can it do something other than Art-Net e.g. sACN/E1.31?
Have a search on the mailing list, as this has come up before. You'll only be able to bind to Art-Net on each IP once, from memory I think if you had two NICs, or simply created a virtual/alias IP on the same physical interface, it should pop up two Art-Net nodes in OLA, so you could then listen on one and transmit on the other.
Although if they're on the same interface, just receiving on universe 1 and transmitting on universe 2 is probably sufficient, you might not even need the second IP. The OLA unicast setting will be Art-Net global I think, but that doesn't actually matter if you only ever need to send as unicast; the receive can still be broadcast.