Using separate command are all right for me.

Note that when you ask for changes to component in the context of a given stream, it will give you all the changes that have led up to the current state of that component in that stream ... all of these changes will have been made in various workspaces (except for the web client, which makes changes directly to a stream), and then delivered to the stream (possibly having been delivered to other streams on their way to being delivered to this stream).   So it doesn't really make sense to talk about "changes that happened on stream B" ... the changes happened in various workspaces, not in stream B ... stream B just "receives" sets of changes from varioius places ... including stream A.

If the changes from Stream A were the base of Stream B then you also get that as part of the history.

For the purpose of what you want, I think you should take a look at the compare command. It will show the differences between the two streams. Unless you've delivered the changes from Stream B back to Stream A, the changes shown for just Stream B will be what you want.