with our small (only one dual port 10Gb fibre LAN card) 5250 we went with:
host0/eth0 used purely for Flex Console
nic3+nic4 bonded using 802.3ad to form bond0
add a "backups VLAN" details of "VLAN ID + subnet/CIDR + gateway" to bond0
when creating Primary Server instance, pair with the "bond0.VLAN" and select an IP in the "backups VLAN"
when creating Media Server instance, pair with the "bond0.VLAN" and select an IP in the "backups VLAN"
so now Primary and Media live in the same "backups VLAN" network
any backup clients in the same VLAN can route to both Primary and Media
and if you need to for backup clients in other VLANs, then you can always add more "VLAN ID + subnet/CIDR + gateway" to bond0, and then "pair" the "bond0.VLANID" with an additional new IP address for the Media Server instance - to create a presence for the media server in other networks for clients to send their backups to - i..e. backup clients in those other VLANs would need to have hosts file entries for the media server IP in that client VLAN