I am still not sure I understand the need for the 2 subnets then
Why not just use 192.168.50.0/24
your switch will have 2 LACP groups,
group 1 goes 2 interfaces on SRX1, this is put in RETH1
group 2 goes 2 interfaces on SRX2, this is also in RETH1 (since they are a cluster)
When the firewall fails over, the secondary will take the Ip address of the current active firewall, for example 192.168.50.1