Can you share a simple drawing of the setup and relevant configuration from the SRX? Will make it much easier to guide you.
It sounds like the Cisco routers are handling failover between ISP A and ISP B and then provide the SRX-cluster with an HSRP/VRRP address for redundancy. Then the SRX cluster should just have a static default route. Putting two reth-interfaces into the same vlan does not make much sense.
You could move the BGP to the SRX cluster but I agree that it doesn't make sense for now.