No, it's not like that.
This is because,
There is no need to do MBBS for doing BDS as you can directly do BDS after your 2nd PU or class 12th.
Admission in both BDS and MBBS is through the same entrance examination i.e neet, you can directly do BDS or MBBS after your 12th through Neet and to be eligible for appearing in NEET examination you need to fulfill its eligibility criteria provided below:-
*Neet 2021 eligibility criteria says that, to be eligible one must be either appearing for the class 12th board examination 2021 or one must have passed 12th board examination with physics, chemistry ,biology/biotechnology and English and must have got at least 50 % in aggregate of PCB i.e Physics, Chemistry and Biology/Biotechnology in 12th examination if belongs to general category , or at least 40% in aggregate of PCB if belongs to Obc/ Sc /st category , or at least 45 % in aggregate of PCB in case of general pwd candidates to be eligible.
* to be eligible for neet 2021 one has to be at least 17 years old or more by 31st December 2021
So, if you fulfill these criteria then you can appear in NEET examination and then can get admission in MBBS or BDS on the basis of rank / marks obtained by you in NEET examination.
For detailed information about neet examination eligibility go through our page for which link is provided below; -