Never ever do that thing ...

Imagin 2 FAT people walking in the same line at the same time and they wanna enter in a building with just 1 door, what do you think will happend if they try to enter at the same time ? ...

It will happend the same between webmin and cpanel, both use Apache, phpmyadmin, php and other softwares and they have different needs and configuration for this softwares

So the point is "Dont install try with cPanel or unistall cpanel and put Webmin"