“What do you mean by two sets of Mitzvos?”
Not a difficult concept to grasp:
Set One- Mitzvos bein adam L’Makom i.e., avoda zara, kashrus, shabbos, shiluach hakan
Set Two- Mitzvos bein adam l’chaveiro, i.e. stealing, arayos, murder, L”H, etc.
The two sets of mitzvos form the ONE Torah. If someone is oveir any of the laws of Set Two, which is also commanded by Hashem with absolutely the same chashivus as Set One, it would be an aveira against both man AND G-d. That would be a double whammy,something which Yom Kippur is not mechapeir without asking mechillah from Hashem AND also from the person you injured in some way, whereas if you eat treif or are mechallel Shabbos, you only need to get mechillah from Hashem.