Loopback IP overlap

I'm deploying Fortimail with 2 VA's in HA config only mode. We do want them behind a loadbalancer. The loadbalancers are in the same subnet for the highest performance.

Subnet on Port 1 is 10.240.240.15/23 and the loadbalancer is on 10.240.240.21/32 When I try to add the loadbalancer IP on the loopback adapter I get the error "IP overlaps with other ports".

Is there a way to add the loopback address in the same subnet?