Question on supplements?

I don't have any information regarding wheat bran or the supplements you're questioning (other than the fact that rice bran is better than wheat bran, and i would never feed anything not intended for food unless I knew 100% it's safety), but I do have a suggestion. Never, ever double up on a multivitamin (or a supplement in general, unless it's a loading dose). You're literally poisoning your horse. Your horse is already getting a good amount of it's vitamins/minerals from its feed (between hay, grain, complete feed, whatever). All-around supplements or multivitamins are normally good to balance everything out. But when you're doubling them, most of those vitamins/minerals just need to be excreted, causing more work for your horse internally, and it can be hard on them. If it has selenium (likely), this is extremely dangerous in too high of doses. So my suggestion is a phosphorous/calcium imbalance is better (for the time being-no imbalance is good long term) than poisoning your horse. My other suggestion-if you have a question about these things, your vet knows these things and can give you advice on them. Talking to your vet about what you're feeding and how to balance it is a much better idea than asking random people over the internet.