Hi Simone,
I am unsure if I have understood the set up correctly but as per my understanding it would not be working as you have mentioned, but with a node (eg 300A ) taking traffic when it is primary and if you have the interfaces on switch 1 failing and you have interface monitoring enabled, it would cause a failover to second node (300B) and thus traffic will start flowing via the second node and thus the second switch. The set up should be something like below;
Below kb shows a diagram with the concept of reth interface the rest of the kb may not be applicable here;
https://kb.juniper.net/InfoCenter/index?page=content&id=KB22474&actp=search
So the set up would have to be SRX A ge-0/0/0 and ge-0/0/4 and from SRX B ge-1/0/0 and ge-1/0/4 forming a reth interface eg; reth0 )and reth 0 connecting to switch 1 from SRX A and switch 2 from SRX B, this with interface monitoring will help you achieve the failover to switch 2 when switch A fails and also if SRX a fails.
HTH