After letting Meraki-support enable the NAT-exemption feature, you can selectively disable NAT per WAN-port and even per VLAN.
Works good. With one customer I habe a direct internet-line on WAN2 with NAT, but on WAN1 the ADSL-router in front of the MX is doing NAT, so I disabled NAT on that port.
You probably don't want to have the "Meraki Passthrough" mode which would work as a L2 bridge on a single WAN and routed mode on the other? That is not supported.
If you found this post helpful, please give it Kudos. If my answer solves your problem, please click Accept as Solution so others can benefit from it.