On the SRX static NAT means the ip address can only be associated with a single other ip address. That is why you are getting the error.
When you need to use the same ip address to forward to two servers on different ports you have to configure separate destination nat rules with the ports specified. Example on page 8 here.
https://kb.juniper.net/library/CUSTOMERSERVICE/technotes/Junos_NAT_Examples.pdf