Two science courses in one semester is def doable. Last sem I took Bio, Organic Chem II, and Physics II plus 2 gen-ed classes and managed to do A in all classes. Took a lot of time and effort though... i barely had any free time during the week to do anything fun but i was still able to hang out on the wkends.
You need to get used to taking a lot of science courses because in dental schools all your classes will be science related courses; if you can't handle it in undergrad, what makes you think you'll be able to do it in d school?