MX100 multiple WAN subnets, single MX port?

[threeonesix]

We have two ISP internet feeds, one in the Internet port and one in port 1. Each is served by a different ISP. On the Internet port we have been assigned (not real public IP's, obviously) 192.168.0.0/29. This provides 5 useable public IP addresses. I need 8 public IP addresses. The ISP wants to assign me another block of 5 addresses, 192.168.10.0/29. This will not work on either of our WAN ports, correct? They can only belong to a single subnet?

If this is correct, why? I'm just thinking about a Windows computer ... you can assign a single ethernet port to multiple subnets in Windows. Seems like a no-brainer for a router.

Is there any way around this issue other than my ISP assigning me a completely new /28 block of addresses?

[KarstenI]

This is typically done differently. Just ask your ISP to route the new subnet to the IP that you have on the MX. If you use HA, the new subnet has to be routed to the virtual IP. After that, you can use this new IP-range in your NAT-configuration on the firewall.