@Holli69 if you change orgs to per device licensing then you can keep the budgets separate without needing separate orgs. This still doesn't allow different levels of MX license (they all have to be either enterprise, advanced or sdwan plus). Could that help?
If WAN1 fails, the non-Meraki site to site VPN will go down.
If you want to make it bulletproof, create a transit VLAN at your data centre. Add an MX for each organisation in VPN concentrator mode. Then you can use AutoVPN within each org, and route between the organisations using a layer 3 gateway (such as the MX250, a layer 3 switch, etc).