In the beginning i tried a new prenatal every few days at my Dr's request trying to find one because they all made me sick. In the end i ended up on Flinstones as well.
Switching vitamins won't harm you or the babies but don't take more than the dose because you could overdose yourself on them. Most prenatal vitamins and vitamins in general, leave enough room for you to make up for whatever they may be lower on, in your diet. At the same time, the dose, if doubled, can be too much. Too much of even good for you things, can be really bad!!!! Just make sure the vitamins you are switching to or supplementing with, have a similar make up so it doesn't cause stomach upset. I take Flintstones sour gummies because I get sick from actual prenatal vitamins. The sour helps with nausea too!