Multiple subnets on a single WAN interface and outgoing NAT

Question

I have 2 public subnets on a WAN interface, say 1.1.1.0/29 and 2.2.2.0/29. 
 I configured the first one as the primary address 1.1.1.2/29 and gateway 1.1.1.1. 
 I configured an alias on the port with address 2.2.2.2/29 
 In a firewall rule, LAN to WAN I want to use the 2.2.2.2 address. The problem is that I actually only have one WAN Gateway for the port. In the firewall rule UI if I try to configure an new 'Gateway Host' to set the primary gateway as '2.2.2.1' I get the following error : "Interface IP and gateway IP address must be in the same network". 
 The problem is that in the port list I only have the port with '1.1.1.2' address, I cannot select the alias port (Portname:0). 
 Is there a workaround ?

lferrara · Accepted Answer

Eddy, 
 Alias is a way to add additional IP (on the same subnet) on a physical interface. You cannot add multiple different IP on the same physical interfaces, unless you use VLAN with different VLAN ID. 
 This is a network limitation and not XG.