The thing you are talking about is probably a brigde course which might be offered after completion of BDS/BAMS/BHMS. This is not official yet, at the government's decision is still pending in order to weigh pros and cons.

And to answer your question, yo u will not be allowed lateral entry into MBBS after completing 2 years in BAMS or BHMS . A bridge course can be done after you complete your degree and crack the entrance. It is proposed to be a three year long course.