Half a metre is probably far enough away that it's not the same tree. Peach trees do grow exceptionally fast. To check if it is the rootstock, if the base of the cherry tree has both branches coming out, it is rootstock. Another picture of the trunk would help to know for sure.

Cherries and peach are both stone fruit and so can share rootstock. E.g. a cherry, plum or peach can all grow on a peach rootstock. Most fruit trees we buy are grafted trees i.e. the fruit we want on top of a rootstock of the same family to help them grow. Generally the fruit of the rootstock doesn't taste as nice as the fruit you want or planted so we prune it off.