Hi! If you apply to a BS/MD Program, is it possible for you to still be accepted into the undergraduate school, but not necessarily the medical school? In other words, if I apply to the Northwestern University to Feinberg School of Medicine BS/MD Program, would I need to submit another application to Northwestern University for undergrad? If this is true, does this apply to all programs or just certain schools? Thanks 
It depends on the school. At Boston University if you are denied admission to their BA/MD program you will not be eligible for regular undergraduate admission. You cannot submit a separate undergraduate application.
Check each school’s website.
In general for any BS/MD programs, first you need to apply to the UG program using commonapps or whatever other options allowed.
Many schools forward the app, transcripts, LOR ets to the BS/MD program evaluation office. But many programs you also need to submit secondary apps for the BS/MD programs.
But for few schools, in addition to the common app, you need to go thru the complete application process including sending the transcripts, LOR directly to the BS/MD office. It is pathetic some of the top schools don’t have a convenient way to share with BS/MD office and expect students to go thru the process and adding more work to HS counselor/office. Northwest and Pitts are 2 such examples and it is a shame to have this for years. They deserve grade F for this.
Coming back to Boston though you apply to the specific BS/MD program, once if they decide not to invite you for an interview, you have the option to inform them to consider for UG. Additional step, but still it is not bad and they give admission. Same true for CWRU.