CBSE 12th

B.Com. is a graduation degree course, and Cabin Crew course is a professional training. It will not add to your education, but it will help you get trained for a particular job role.
Hence there is no harm in doing both simultaneously.
For your convenience, I've listed below the general guidelines and requisites for becoming a cabin crew member.
In general, candidate's profile for freshers for cabin crew is as under-
Age: Between 18 and 27 years
Minimum Height: Men - 170 cms and Women - 157 cms
Weight: In proportion to height
Educational Qualification: HSC (10+2). Candidates who are appearing for their HSC examinations can also apply.
Physical features: Pleasing personality, clear complexion (scars, pimples and blemishes not acceptable) and good eyesight
Marital Status: Unmarried
Language skills: Fluency in English and Hindi. Proficiency in a foreign language will be an added advantage.
Interactive skills: Excellent communication and good interpersonal skills
Nationality: Indian citizens only
Passport: Must possess a valid Indian passport or should have applied for a passport
Mandatory Documentation: Must possess a Pan Card and Aadhaar Card
So that becomes very clear that it merely requires some grooming to get the job of a cabin crew. They don't demand much from the candidates.
You should be interpersonally sound and capable to handle people from different cultures.
Cabin crew is the face of the airline.
Foreign languages like German, French, Russian, Arabic, Chinese and Japanese, etc. can allow you to get an edge over others, as this allows the airline to roster you on routes where such languages are spoken.