I thought as siptv is m3u-based, you can use the same playlist on multiple installations of the app (not at the same time obviously). Only downside is you'd have to pay for siptv once on the upstairs fire stick. But upload the playlist afterwards using it's new MAC and it should work, surely, without knocking the other one off. And if that doesn't work, creating a tinyurl link for the m3u should trick it into thinking it's a different playlist.
Failing that, I would recommend emulators of mag architecture (i.e. stbemu) over the standard android APKs although that's a personal hatred towards smarters-type apps. The macs chan be changed to match each other in stb.