You would be interested to read this blog SMP 3.0 : Accessing more than one OData services in one application for your current use-case.
To retrieve resources from 1st OData service, you have to call,
http(s)://smp:port/appid (of-course, you have on-board user first and pass its AppCID in the header section of the GET request)
To retrieve resources from 2nd OData service, you have to call,
http(s)://smp:port/connectionNameFor2ndService
and so on.
Regards,
JK