If they are two separate sites, why would you want to keep the same IP address across them? You’re not going to achieve client roaming between disparate sites. Is there a reason you want to keep the same IP address?
For the Layer 3 roaming to work the APs would need to be within the same Meraki network. Generally if these are two separate sites then you’d create a Meraki network for each site, so roaming wouldn’t work. You could create a single wireless network that spans both sites, but it’s really not ideal, and you need to think about the use case carefully.
If you really want keep IP addresses on the wireless networks at both sites the same then you could use a wireless Concentrator (a Meraki MX in VPN Concentrator mode). Then the SSID could be tunnelled back to the concentrator so the IP subnet/DHCP is assigned by a VLAN accessible from the wireless Concentrator. Note that this won’t achieve roaming between the Meraki networks, a full re-Auth will happen when they move between them, but it will mean that the same IP address range can be used. You also have to be sure that you understand the traffic flows (I.e. they are via the concentrator) so that you minimise traffic hair-pinning.
If you haven’t already, have a read through this document, https://documentation.meraki.com/Architectures_and_Best_Practices/Cisco_Meraki_Best_Practice_Design/...
