It all depends on the recruitment procedure, syllabus, and level of the examination. When all these three parameters are similar for at least about 75%, then it is worth giving it a try. If any of the parameters are completely different, then you should choose only one exam that you believe you will have a better chance of success.
For example, if you are preparing for UPSC Civil services, then you might also want to give APPSC/State Group 1 a chance. Then consider the following things before making a decision.
Similarities
The Recruitment procedure, syllabus, and level of the examination are almost the same for these examinations.
Differences
| UPSC Civil services | APPSC or any State Group 1 |
| The syllabus generally does not comprise state-level topics. It focuses more on national-level topics. | The syllabus also comprises state-level topics. That means you should spend extra time to cover these additional topics. |
| The pattern of questions changes from time to time. The questions are mostly analytical and difficult. | The examination procedure and the format of the questions are as uniform as possible. The questions are less analytical, a little easier. |