You can hold on to your Nepal passport. It is perfectly legal to do so. You cannot use both as Nepal does not allow dual citizenship, so holding on to it is fine as you would not use it anyways. For example, if you went to Nepal on a US passport, you will have to return on that same passport.
It only becomes illegal if you use it. In future, when you decide to revert back to Nepali passport, the process will be easier for u.