Four channel is not required but your choices will be a little limited with the two channel.

The Mixmaster can split the banks into positive and negative fired groups and also provide a trigger.  This requires three channels on the scope.  One for one bank, a second for the other bank and a third for trigger.

You can use the Mixmaster with a two channel scope but you will have to make a choice between splitting the banks and using trigger.  You can choose not to split the banks and use trigger, or split the banks and not trigger.