Unfortunately, subnets are not allowed only ip addresses.
This is based on the assumption (not correct here) that the calls will be made from some sort of permanent server platform. So that is one option, setup a common VM that all in both teams can use to initiate the REST calls.
The other option is to insert some kind of NAT in the path between these subnets and the SRX that can change the source address for these calls to a common single shared address.