MTG: march of Multitudes and Early Harvest
Yes, if you do things in the right order. While you are casting a spell and paying the cost, you don’t have priority to do something like cast Early Harvest, so you cannot do this if you start to cast March of the Multitudes first. By the time you would be able to cast Early Harvest, March of the Multitudes would be finished being cast and couldn’t be changed afterwards. 

However, you can tap all your lands first, producing some amount of mana; then cast Early Harvest; then tap all your lands again. This will let you use all of your lands twice to have double your normal amount of mana (minus the 3 you spent on Early Harvest). All of this mana will stay until the end of the current step; so you can use it to cast March of the Multitudes.