Hello! I've finally received an answer from Eric. Here's his answer:
I finally, FINALLY have good news to report. We just published an update to TubePress Express that allows you to utilize multiple sources in a single gallery. At last! I sincerely apologize for how long this has taken us. Thing about software is, things that seem like they’re easy to do often times can be very difficult.
Give it a try and let me know how it works for you? I wouldn’t be surprised if there is a bug or two that we haven’t found, so please feel free to report any trouble.
Thanks again for your patience with us!
I've just tried with one of my galleries and it's working perfectly, so I'll mark this topic as solvedGo to the full post