You can pretty much pick when the horse gets the supplement, as long as you're fairly consistent with it. Horses love consistency. So yes, if you want to split it between feedings, I don't see a real problem. Hoover takes Farnam Platform for his pasterns, and gets his scoop in the morning, but occasionally we forget and give it in the evening.
What kind of joint supplement is your guy getting?
DGW- I agree. The thing to look for in a supplement is making as natural as possible, like with human supplements. The body can process naturally occurring minerals and vitamins much better than fabricated ones. So, for example, go for a supplement that gets it's glucosamine and condriotin from shellfish or shark cartilage rather than a lab.
"Sit tall in the saddle, hold your head up high. Keep your eyes fixed where the trail meets the sky. And live like you ain't afraid to die...don't be scared, jut enjoy the ride." - Chris LeDoux