Yah-E
You are a bit disconnected from reality I believe on this...not just some schools will think it is a negative; all of them will. No school wants to invest in someone who is so unsure of what they want to do that they are applying to two completely world-apart (let's be honest here) professions. Both professions require that you know for sure what you want to do so that they arent making a bad investment of resources. Why would a dental school accept someone that is not sure they want to go to dentistry over someone who is? Many would even say that the whole interview process is to find out who most WANTS to be a dentist because this will naturally make someone work hard to achieve his/her goals. I would say with relative certainty that in most cases, when a dental school has to choose between two equal (in GPA+DAT) candidates, the one they will choose is the one they think is most completely and solidly committed to a dental career. Someone who is applying to both types of schools clearly is not one of these applicants, and thus will not (in most cases) be given a high priority in acceptance. I would never advise anyone to apply to both types of school, knowing what I do now about the process.
Guest,
Through all of this I am not saying you wont get in anywhere but my advice would be to choose one before you apply to both; spend enough time in both settings so that you can decide what you want your life's work to be. Deciding what you want to do with your life AFTER the application process will make your life a lot tougher. It will make you a less serious appearing candidate, make your life very difficult at any interviews you get, cost you a ridiculous amount of money (MCAT+DAT fees +application fees+interview trips, etc.), and will also put you through a lot of unnecessary stress.
I used to think medicine was great, then I decided to go into something really interesting-dentistry. Maybe the same will go for you, maybe not, but make those decisions before you apply!!