My fish have Velvet & white spot. Why do the medicine's (King British)say not to be used with other medicines?

I added both medicines to the tank, KIng British velvet control yesterday, King british original formular WS3 white spot terminator today. Now my fish are acting strange, there all hiding behind the filter, under the tree stump. Only the sucky(sorry don't know what it is) is out playing! Any help?