Why don't you just do that now and work half the amount of hours you're currently working?
Show me the job like mine where this is an option, and I'll take it in a second. Hire another me and we'll split duties.
These sorts of "professional job that pays a professional hourly rate but is for 20 hours a week" are exceedingly rare. You'll usually be taking far less than 50% pay - far worse if you include benefits in the calculation.
I've been halfway keeping my eye open for such an opportunity so I could fund the basics of my life, plus have time to do personal projects with utterly no chance of monetary payback. Just stuff like paint the house, teach myself how to weld, work on backyard art, volunteer, etc.
I could certainly find a job that pays 50% of what I get now for working the same number of hours though. Perhaps moderately less stress and no "off hours" chance of being called in for an emergency. But that's not a great tradeoff since I'm looking to trade money for time.
This may not be the point you're making, but it really is sort of frustrating this isn't an option. I get why - I employ folks too and understand the overheads involved - but man it's the dream!
And that's my point.
Your purchasing power will not change.
Your purchasing power is defined in a competitive equilibrium with your peers.
If you're assuming you can band everyone together to all decide to work fewer hours for the same pay, fine, but you just invented a union, not an improvement to UBI.