grif's right: I've only ever mixed the Ca2+ and vit for reptiles and there's no harm in separating the two even for them. For my juvie fantasy, I dust my crickets/worms with Ca2+ w/o D3 5x a week then Saturday with Ca2+ w/D3 and Sunday with herpivite. I believe the vitamin powder has D3 in it, but 2 days a week with D3 isn't going to hurt anyone. I plan to continue this regimen until he/she is fully grown. Keep in mind that there's always the danger of toxicity with any sort of supplement, so don't overdo it with the D3: calcium toxicity is just as bad as calcium deficiency and can actually cause death/coma more quickly. As grif said, proper Ca2+ helps with hydration, but too much can actually dehydrate them. They're nocturnal and produce their own D3 anyway, I just give it because I don't provide UVB and it makes me feel better.