See... that's the big thing: I wouldn't trade action economy. Just time-economy. Both people would still get their 'attacks' off as if they spent their action. But delaying and working together could give them a secondary benefit. Like turning a lightning bolt around a corner, or ignoring damage resistance.It's a fun thing I've seen done in video games, but I get the feeling that it might be hard to set them up in a TTRPG; plus, they'd have to be dramatically powerful, due to the 2:1 action economy. Which I don't mind, but some GM's might find intimidating.
SOMEONE has to delay, SOMEONE has to be in position, etc.