So, the dual degree isn't the MSTP, where there are both financial (tuition is waived) and admission (MSTP admissions are notoriously easier than regular, and a lot of the MSTP admissions is looking out for the fakers who go into MSTP to get easier admission and then drop out). What happens to shorten the time is instead of taking the PharmD version of the classes, you'll instead take the graduate form of the class which means for your PharmD portion, certain classes get much tougher without respite from the other clinical ones and the fact that you'll be tending lab. It's fairly stressful for the dual degree, and they have to work really hard, but they do shave off a year or two from their training, although none of them that I have known have gone into practice (to lead me to ask them WTH did you get the PharmD anyway unless you're a Clinical Trials major where the PharmD is given some regulatory privileges.)
https://www.nigms.nih.gov/Training/InstPredoc/Pages/PredocInst-MSTP.aspx
It's not going to matter either way, you'll get admiited. There's a low conversion rate to the dual degree program as it's quite a bit more expensive to undertake than undergraduate. It's also highly dependent on your graduate major as well.
If Pharmaceutics, I hope you have Physical Chemistry (which has an implicit background of a full year of Calculus and some experience with Differential) as a prerequisite to a couple of classes (the ones that involve solubility and diffusion).
If Pharmacokinetics, you must have had Ordinary Differential as the very first class using the Loftsson or the Gibaldi assume you have it. You'll need to get a class in numerical methods during the program unless you've had it as a prerequisite.
If Pharmacology/Med Chem, it's very dependent on the school, but it's nice to have some knowledge of Animal Physiology as well as the full year of Human as well as Analytic Chemistry at the Skoog level. You should seek out your prospective laboratories and figure that out what the actual and implicit requirements for the laboratory are, since the field is diverse.
If Clinical Trials or Regulatory, I basically would say if you have the background to read Clinical Trials by Piantadosi and follow the math and statistical reasoning, you'll be fine (some Calculus, some linear algebra, some statistics).
If Biostatistics, you will need both Real Analysis (Baby Rudin or equivalent) and Measure Theory (Tao or one of the Russians) to take on the Jun Shao Mathematical Statistics book.