Hi There
This decision depends on estimating your capability.
Analyse the Pros & Cons of writing Group wise and Both groups at the same time.
Check whether you spend adequate time to prepare for both B.com as well as CA considering Articleship.
To my mind Writing Both groups at the same time is better,since it gives u great confidence.
Also Surplus marks in one Group will help to clear,that is not possible if Group wise exams are taken.For e.g Both Groups (G-1=120 & G-2=180) is still treated as Pass if both groups are written at a time,whereas writing Group wise If one Get Group - 120 by getting 40,40,40 still he is flunked.