If you have both the Fibaros in the same Group (Bedroom Lights) in Alexa itself does it not turn them both off anyway?
Probably the easiest way is to create a Virtual or Simulated Switch in the IDE, go to My Devices, Add Device,Type in a Name (the Name will be in ST as a switch), enter a unique Network ID (make something up that will not be replicated anywhere), select a Type of “Simulated Switch” (You could try “Simulated Colour Control” but do not think it will work with Smart Lighting so experiment once you have the ON/OFF working). Set the Location and Hub, click “Create” This will then give you a Virtual or Simulated Switch in ST. Then use Smart Lighting or similar App with this new switch to Turn both the Fibaro RGBW’s on/off, again not sure how the colour will work here but play around with the Simulated Types and different Apps.
Can also take a look at the Trend Setter App, you should be able to switch them ON/OFF under one switch with this and possibly even change colours although the scenes will obviously not work. Add the Switch and Colourful Device, the app and child app first and try to Colourful DH on the Virtual switch first and see if that works, if not then try the simple Switch to just turn them ON/OFF together. I assume this app does pretty much what I detailed above but the DH’s may be better than the delivered ones and give you colour control on the Firaros as well but remember this is not the Fibaro DH so it will be basic controls of course, if it does work. Have not tried it myself but it may work.
https://community.smartthings.com/t/release-trend-setter/31286
I would also take a look at CoRE and/or webCoRE, steep learning curve but you can pretty much achieve anything with those.
Saying all this cannot see why the amplifier would not work as at the end of the day it is just a normal RGBW powered strip and the controller is irrelevant. This will probably solve all the issues leaving you with just one Fibaro so if you have no need to control them separately I would take this route to save you messing about with Virtual switches and or CoRE etc…
Let us know how you get on!
Best of luck!