I thought your body stores extra vitamins and minerals in your bones and fat, but I may be wrong. You should check to make sure your multivitamin doesn't already contain biotin. Pretty sure the recommended daily amount is around 400 mg
Generally it's fine, especially if they don't have the same things in them.
You do want to be careful with taking too much iron.
Quoting FKSC;[EBFT]:" Is it okay to take multiple vitamins at the same time? I regularly take one a day womens multi vitamin. ... [snip!] ... multi vitamin. But today I also took acidophilus and biotin. Is it true your body pees out extra nutrients it doesn't need?"
Your body only stores a specific amount of the vitamin so yes once you start taking in too much you will pee out the extra!
edit- you can actually overdose on every vitamin but you should be fine because with our diets today it really isnt common lol
Your body will get rid of extra of certian vitamins but not others. I had a very bad reaction to taking the recomended anount of B3 and was told at the hospital that it was an over dose and could also cause kidney falure if it was bad enough.