It's possible, but the RX 570 card already has a hardware encoder which would affect the performance of the games by a very little amount, almost negligible.
It would make sense if you have a second PC and use that for uploading the stream... basically you'd play on main pc with rx 570 and use OBS with NDI plugin, which then pushed the raw stream to 2nd pc which compresses everything.
The 5450 would be good enough for 1080p 30fps, maybe even 60fps with some relaxed quality preset (as in set to "fast" or "medium" quality)
The card would be useful to have around, for testing other computers, if your friends bring you a dead computer or something, it still has vga output which the rx 570 doesn't so if you don't have a processor with integrated graphics it could be useful to use with vga only monitors..
It also consumes little power (3w in 2d/idle/windows, maybe around 5w when watching movies, 13w when gaming) and you can tweak it from control center to use even less power
I would consider it for a very low power pc, like for example something to mount in the back of a monitor (on the vesa mount) and powered from 12v only... think let's say an Atom pc with 4 GB of memory and the video card... it would be better than the integrated card in the Atom.
It would also be good enough (hardware feature wise) to work with various emulators (snes, ps2/ps3/nintendo etc) so could be used for some gaming by the tv... make your own "console".