I hope you are aware that a redundant Ethernet Connection with a node like a S7-300 is not possible. So I wonder what type of connection you try to establish?

If I think about it, it should be possible to work with one singe Ethernet Interface with a Switch in front of itbut you may have to handle two connections one form each of the S7-400 H systems. The program in your S7-300 has to handle both connections and you have to handle the switch over between CPU A to CPU B within your S7-300 PLC. If you care about all this... it should work.

I hope it does in your case.

Best regards
J_Bell