1:1 NAT multiple external IP addresses different interfaces to 1 intl address

[klubar]

I have 2 external interfaces (call them A & B) in a failover load balancing group; if I set up a 1:1 nat from an external address in A and an external address in B to the same internal address (192.168.10.46), when the device at 192.168.10.46 accesses the internet how does it "know" whether to use interface A or B?

Is it based on the order of the 1:1 NATs, or based on the priority of the load balancing group, or something else?