Thanks Icube !
Quick observation before troubleshooting further, you're missing the hashing for layer-3. Paste the following command into both SRXs configuration mode :
set forwarding-options hash-key family inet layer-3
And if you could share the routing tables for both devices ( operational mode ) :
> show route
After that try your test again; but this time instead of checking the paris-tracert output issue this command on the srx connected to the client you are testing from ( R1 ?) :
> show security flow session
You should be able to see all the open sessions on the firewall and it should show that the outgoing interfaces for each session is different from the one before it, rotating between fe-0/0/2 and fe-0/0/3 .
Can you check that and share it with us ?