this is surely possible but as you may imagine, you have to face some problems here. The most important problem is that you have on the one hand 2 CPU's with Ethernet CP's and on the other hand you have one PLC CPU with one CP.

So on the side with the single CPU you have to use 2 Connections and on the H System side you have one connection for each partial system. Within the H System you have also to detect which one of the PLC’s is on right now.... to decide which connection to choose... well, just in case you have different Connection IDs.
Also, after a switch over between the CPU's you have to ensure the reset of the last task, if it is not finished yet... a.s.o.

If you would like I would recommend the use of S7-Connections with PUT / GET from the S7-400 H side. This way you do not need to handle anything by the single sided CPU. Everything is being done by the PLC OS.

It is just an idea.

Best regards