I'm assuming you are talking about going to the network object itself and configuring its NAT tab. If you look at the 2 automatic NAT rules generated as a result in the NAT policy, the destination of the second generated rule which does the vast majority of the NATting for that network has a destination of "Any". So yes it will NAT that traffic to all other interfaces including the second internal one. Typically you would have a manual anti-NAT/no-NAT rule defined early in the NAT policy that will disable NATting between internal networks and/or DMZs. The first auto-generated rule specifies no-NAT for hairpin/u-turn situations involving that network and is rarely hit.
I think the checkbox Val is referring to is located on the gateway/cluster object itself on the NAT screen. If you check that one yes only traffic exiting on the External interface will be NATted.
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