Yeah if we can manage to earn from both of them then we should work on it as your earnings will be higher if you combine the efforts which will increase your wealth.
Thats why proper time management is really needed when you decide to have two jobs to do. You could earn decent amounts for sure if you do both jobs but you would need to double the effort as same as other said but if you think that you couldnt handle both then dont bother to do it.
Yes you can go for both only if you can manage to do that and if you are willing to put some extra efforts in earning money and if not then its better to continue with full time job.