Can an RX 550 run 2 Monitors? Technically, Yes.
Should you use it for such? ABSOLUTELY NOT
Strictly speaking., all GCN / RDNA Based GPUs (even Integrated Graphics., like in the A-Series or Ryzen with Graphics) are capable of supporting a minimum of 3 Displays.
This functionality was added as part of the Eyefinity Feature of the Display Engine (UVD) in the Terascale Architecture., so technically all AMD GPUs (HD 4000 Series) onward are technically capable or running Multiple Displays.
The problem at play here though is Pixel Throughput.
In basic terms the RX 550 has just enough Pixel Throughput to handle a Single 1080p Display at 60Hz... you _might_ get away with 2x 720p Display at 50Hz but I can't imagine it would be able to do much more than that.
Honestly., if you want to have a Multi-Display Setup (and want to do more than just use the secondary display for you Music Player, Web Browser, etc. while doing Work in Office on the other) then you're going to want to look into investing in a better Graphics Card.
I'd recommend the RX 470 / 570 / 480 / 580... as I know from experience they can handle (comfortably) 2x 1440p60 Display., or say 2x 1080p60 while gaming on one.
Of course keep in mind that ANY Additional Display will actually affect the performance of any game your running.
So let's say that you would normally run say Fortnight at 1440p60 High Settings (which the RX 480/580 can do) ... well add a Second Display to the Mix that's rendering a Desktop at say 1080p60, then you're going to end up needing to go down to say 1080p on the main display running Fortnight to maintain a Stable 60Hz.
That isn't to say it affects performance how you'd think... technically the game will still hit the same Highs and Lows as before. The issue actually stems from the fact that Multiple Displays means you're adding another Render Queue, and so this will affect a game' Avg. and Frame Jitter.
Streaming can (depending on CPU) also do this.
Honestly speaking., it's actually better NOT to Stream from the same PC as you're gaming from... not unless it's a bit of a "Beast" so-to-speak.
Bare Minimum you'll want a 6 Core CPU, plus a GPU with good real-time encoding (again RX 470 / GTX 1050Ti for example)... as remember it isn't just YOUR experience that's important for Streaming but rather how good the Audience Experience is.
An underpowered CPU without enough Threads to Game and Stream... or a GPU that can't keep pace with Real-Time Encoding while Gaming; and well the end result is going to be a block compressed, frame dropping and jittery stream; which is just a crap experience for everyone watching.
As a note you can pick up a Ryzen 5 1600 (Original) for like £100 now., and an RX 580 for about the same.
That more or less would solve any hardware concerns for streaming; but then there's still Internet Connectivity to consider.
I've seen far too many Streamers using WiFi Connections and/or Low Bandwidth Uploading; resulting in just some terrible initial compression... then factor in whatever Streaming Services Compression and it Lowers that Quality even further. Arguably it's why many "Pro" Streamers use a cheap-and-cheerful Secondary PC., or just have a "God Rig" with fairly expensive internet connections.
Yes, I know there are "Guides" and Support Forums to help get people starting with less capable solutions., but unless you're just doing it for the giggles and do expect to grow an audience; you kind of do need to invest properly.
Half Keistering it just isn't going to cut it nowadays, because competition is much fiercer with deeper pockets.
