I faced a similar situation when we added new cabinets to our kitchen. Our vent was in the floor, but, more conveniently, didn't span 2 cabinets.
I simply purchased some rectangular sheet metal ducting, cut a hole in the bottom to allow the air from the floor duct in, and ran it to a hole I cut in the toe kick. I put the vent from the floor duct into the hole in the toe kick. It's served admirably for the last 30 years.
I would suggest that you can do the same, except that you'll have to split the duct lengthwise and put 1/2 into each of 2 cabinets. I see no reason why you couldn't simply attach the duct to the cabinet sides to help ensure that the majority of the air actually makes it to the toe kick instead of leaking out under the cabinet. You'll also have to come up with some sort of custom grill to cover each half as it pokes out from below two neighboring cabinets.
To be honest, you could simply allow the air to blow under the cabinet without any ducting at all, but you will lose all air speed and it will just dribble out of the vent(s) in the toe kick. It will still heat/cool, but it wouldn't be as efficient at getting the air into the living space instead of just being trapped under the floor. This isn't a highly recommended option, but it is an option.
Due to the added complexity of ducting across the two cabinets, you might consider rearranging the cabinets to allow the vent to fall under just one cabinet. Obviously, you've already purchased the cabinetry and have your (spouse's?) heart set on this arrangement, but... it might make your life enough easier to make it worth it. Something to consider.
An option that would involve more work, yet make life easier (ah, the oxymoron...) would be to gain access to the duct work, most likely from below, and reroute it slightly so that it exits the wall or floor a bit to the left or right so that it is below one cabinet instead of split across two.
Since this will be hidden behind cabinetry, you could also tear up the wall/floor a bit to gain access from above to reroute it there. If your house is newer, it's likely that flex duct was used, so there's a chance that there will be enough room to wiggle the duct around to make it come out where you now need it.