Why did ghana king practice both islam and their own traditional religion

To win the hearts of their islam subjects, and to stay in touch with their beliefs.
hope this helps you

They didn't practice Islam at the same time. Once Islam spread, they converted. They also kept some distinctive touches from their traditional religions though. They did not start as Muslims since Islam had not reached them yet. They only converted to Islam because they were convinced it was correct (Which in my opinion, is a correct religion). They kept a couple touches of their original religion out of loyalty to their history, culture, and those Ghanians who had not converted to Islam.