I just googled "68th MO Inf" and came up with that EMM article. I didn't pursue it any further, but note that your ancestor is not listed on the roster there. As I said, there could be a number of explanations.
Maybe there was also a 68th MO in Confederate service (if so, I didn't see it). Or, maybe your ancestor served on both sides at different times. This actually happened to some boys who were captured and paroled or pardoned early in the way. They went home, intending to honor their paroles and then were forced to join the EMM. This nearly happened to the Quantrill man, John McCorckle. He went home to live on his uncle's farm and abide by his parole. When he was ordered to join the EMM, he went to the bush and joined Quantrill instead. Perhaps your ancestor was in a similar fix and saw no recourse but to join the militia.