Dear @jonas,
Thank you for the detailed explanation.
For problem 1: There are no ranges to sigma because I want them to be fixed and I have also made changes to the ranges.
For Problem 2: I have tried to fix it in a different way. In the input root file that I am using. The different trees are still there but the names of the variables are same in all the 3 trees are the same. I think this fixes the problem.
For Problem 3: But, I want to fit for the total yield with exactly the same PDF for D0 and D0bar samples.
I am sharing here the upated folderNew_simultaneous_fit.tar.gz (2.6 MB)
in which I am working. I think the fit is working correctly now. The fit converges when all Araw parameters are fixed to 0 but the pulls are not ok.
Eventually, I need to free all the Araw parameters and I also need to have the pulls to be ok.
Please let me know what you think. Also, I wish you a great year ahead 
.
Regards,
Sanjeeda