Opening same author's another application without URI?

SO has many threads to answer - how to open another application from my application? or How to use URI to open another application etc. But I am referring to the different scenerio.

I have App A and App B, both installed with my signature and I want to open an activity of App B, in the same task stack of App A and return some processed data back to App A.

Lets think it as the library of facebook, paypal etc, and how their library (SDK) can open their application. I know using URI and task affinity, but I am looking if any other way exists in android to open other application (with same signature, that means allowed to have all permission from that application).

Can you please suggest whether facebook paypal etc. also do use the intent with URI to open application from the library (SDK)? Any suggestions appreciated.