Possible to drop Lan access to pfsense when Wan is down?

[Wombly]

I know this sounds crazy…

I use this second pfsense box as a 'slave' to the 'master' box
the master box(192.168.75.1) is monitoring the gateway ip (192.168.20.1) of the slave box..so i want the master box to 'see' that that the wan in slave is down = will not answer to pings (slave is monitoring an ip on the internet)

I  use wan balancing on the master and the monitor ip has to be the same on wan1 and wan2 (and pfsense forbid that..so that is why i use second box and  monitoring the ip of the slave gateway instead)

(I cant use 8.8.8.8 or 8.8.4.4 or others. =They are blocked  so i have to use this specific monitor ip that only works from this network)

Thanks in advance