Hi,
Here is an example of how to acheive this using policy-options in BGP :-
http://www.juniper.net/techpubs/en_US/junos15.1/topics/example/bgp-multipath-unequal.html
Please test if this works for you on the SRX.
Regards,
Sahil Sharma
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Please mark my solution as accepted if it helped, Kudos are appreciated as well.