A fair bit out of your budget but.
2 Sonos Amps, one in each location.
2 pairs of decent quality outdoor speakers. Would have a bit better survivability in a fab shop environment. Them being designed to keep the elements out will help keep fun stuff like grinding dust out of your speakers.
With a Sonos setup you would wire each room seperately (2x speakers, 1x amp). If you want them together you just group them in the app.
Another positive to a streaming setup like this is you don't have to tie up your phone to listen to music, you just use it to control Sonos, not stream to it (although you can if you want to).
Barring that, look for secondhand stereo receivers with A+B speaker outputs. Marketplace, ebay, goodwill etc. Tons of them out there that are no longer valid for home theater use (lack HDMI) but would be perfectly functional in your application.
Those and whatever bookshelf speakers you can find. Buy cheap as eventually they will get ruined by grinding dust and the elements.
Add a cheapie bluetooth to stereo receiver and you are up and running.
Downsides are you would have to wire both pairs of speakers back to one place, you will have to manually turn the stereo on and off and you lose music if you have to take a call.
I have a pile of no longer current but still functional AV receivers here (I am in the AV business). Would give you one if you weren't on the other side of the country.