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.
• Make a schedule giving proper time to both the exams and their subjects.
• The current affairs of both the exams are very different, so make sure that along with reading it conceptually, you are also covering the factual current affairs for MPPSC exam. Pay special attention to MP specific current affairs.
• Understand that the mains paper of MPPSC exam is also based on factual information, so don't ignore the facts, unlike UPSC exam, the questions asked in MPPSC exam are straight and straightforward.
• Attempt enough mock tests and previous year papers of both the exams, and don't forget to revise them regularly. (Yes, revise the solved papers, it is important).
Preparing for the MPPSC as an absolute beginner by Best mppsc coaching in indore
• Always keep MPPSC syllabus and mppsc previous year papers with you so that you constantly analyse the MPPSC pattern and keep yourself aligned with that.
• Start with reading Basic books like NCERTs etc again and again to ensure deeper understanding.
• Follow newspapers (like The Hindu etc) religiously to evolve understanding about issues.
• Follow websites like sharmaacademy, mppsccoachinginindore.com, freemppscnotes.com etc to see daily questions and current affair materials to ensure your views, opinions and thoughts are aligned with UPSC.
• Focus should be on developing an understanding of basic issues and notes making etc could wait till you have a better understanding of topics and issues.
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.