This is a current regulated buck regulator. I don't see much issue with it sharing load. Voltage regulated ones probably won't share well. To share control you could just split out the connection from Cf2 to iadj. So you could possibly make a few boards, only populate the mcu on one, and pull off a wire from that cap to the others.
It might (not sure) be more sensible to think about beefing up the components and cooling, or, yeah, just buying a buck made for it, with a sufficient heat sink, and use a pot to control it as TA said. That would surely be cheaper and easier.
450W is some serious business...that means either extreme amps or high voltage. High voltage gets you there with a much smaller and efficient converter---one that could actually fit in a light, but even if you went with something crazy like 12S you'd be at ~10A, but at 40V+ you've got other issues to deal with. Not impossible, but not something to be taken lightly. A short circuit in a light like that could have disastrous consequences.
Update on that cheap $5 analog current/voltage controlled buck from a page or so back. The current control suddenly went out just while running almost untouched (mild movement of the whole setup) at about 18W throughput. Fortunately I had set a voltage limit that cut in just a tad higher or I would have destroyed four nice LEDs. Turned out one of the knock-off-branded copies of bourns trimpots just quit and failed in the wrong direction. It couldn't be set after that either. The screw did nothing. I tested with an ohm-meter to be sure. While testing though I was pretty sure I also managed to fry the op-amp it was connected to. No big deal since I was going to be ordering parts anyway. I replaced the pots and the op-amp at the same time just to be sure, and all works basically fine now considering the price. It looks like many of these cheap bucks use cheap pots, so it would be worth considering to pick up some bourns pots as a planned upgrade from the start. That makes them a bit less cheap.
Seems to be a bit of a case of getting what you pay for.
Yes, it can do that with the right components installed. At least in theory. I am not aware of anyone that has built one yet. To build a single driver would be like $40 IIRC.