There is no real definition for 'mesh', vendors cobble up their own -incompatible- implementations. Aside from very rare exceptions (of vendors implementing 802.11s, which I think google wifi might be doing), there's little- to no chance to get this to interoperate.
If they are connected by wire, you could use the AX3600 as Main Router, and AX1800 as repeater, and use both of them with the same SSID.
But roaming between AP's will be totally depend on your Client...