This question has come in the mind of every civil services aspirant.
This is easy, firstly study effectively those subjects which are similar in both, UPSC demands more effectiveness and efficiency in handling the subjects under the syllabus. By effectiveness I mean you should study them deeply and understand the concept. In case, generally PSC is more concerned about facts and figures. It has less requirement of concept unlike UPSC. I personally believe that if you want to prepare for both the exams together, then you should study according to the UPSC guidelines as the syllabus of UPSC has a lot more subjects than MPPSC, and a good understanding with the issues will always make it a permanent memory.
UPSC coaching in indore suggest aspirants to prepare for mppsc exam together because your 50% preparation will be done with the upsc exam preparation. When you are preparing for UPSC, I feel that maximum parts of your MPPSC syllabus are adequately covered. If something remains untouched, you can put extra effort for that before the exam. Also, UPSC syllabus covers maximum PSC syllabus but not vice versa though their pattern is different.
I summarizing a basic strategy to prepare for MPPSC exam with CSE in the following points:
Study the syllabus of both the exams and understand the similarities and topics that are similar.
Preparing for the MPPSC as an absolute beginner by Best mppsc coaching in indore
In the end, you must understand that juggling between exams that are very different in their nature will not be easy, but I assure you that it can be done and you should not lose hope.