Using 2 iMacs with new Mac mini

likely unless luna is limited to 1 display their site would probably tell if that was the case

otherwise you could look into some of the other non target display options

Use iMac as monitor: 5 Solutions - 9to5Mac

but

1. you still need a "normal" monitor connected to the mac mini
2. the imacs will use as much power as if they were just being used as normal computers which is a lot! more power than a dedicated monitor uses.

Regardless of how great the folks at Luna Display say their product is, bottom line is it is really a remote desktop app that relies on a network for operation which in the end limits the usability of the product. If you have ethernet or a super fast WiFi it may perform sufficiently for your purpose but will likely still be laggy for very graphics intensive uses, especially if you have any other traffic on your local network.

However, as previously stated the iMac will be consuming the full power of a computer which is much more than just a dedicated display.