Will 2 year gap between 12 and completing graduation in B.Com (Hons) affect my placement after completing my MBA.

No, it will not impact your placements. You just need to be able to convince the employer with a valid reason for the gap. So, don't bother and focus on your MBA.

If you score really well and you have done B.Com then it might fill the gap and not affect your placement but this is something which will be questioned and you should be ready with a valid answer.