As a starting point, it’s a good idea to consider the ‘core’ subjects of maths, physics, chemistry and biology. If you do not have a clear idea of which HE STEM course, or courses, you are interested in applying to then studying two of theses ‘core’ subjects will help keep your options open. This is fine if the subjects you’re eliminating are not ones that you are likely to need or be interested in.
Maths and physics is a good combination for many HE courses, including engineering, geophysics, astronomy/space science, nanotechnology, naval architecture, materials science and meteorology, to name just a few.
Chemistry and biology are good combinations for areas such as biotechnology, medicine, genetics, dentistry, forensic science and dietetics.
Most of the times subject combination are taken as PCM or PCB along with a language and a fifth subject which could be Computer Science, Engineering Drawing, Economics, etc.
Definetly taking PCM along biology will have better carrier opportunities as you can opt for both medical or engineering stream.
If difficulties are concerned than it will be upto you, but if you are particularly intrested in engineering or medical than take admission according to your interest.
Just to clear 12th standard having both biology and maths is not really hard if you have proper understanding of both the subjects till 10th standard or else leave the one in which you are not good.
Also if you want to aim for some competitive exams on any side like AIEEE or IIT from maths side or PMT or any state medical entrance test along with 12th standard studies then i would say having both the subjects is not at all a good idea and it will become very tough for you to study.