Multi select isn't possible in a parameter. Filters are your only user friendly way of multi select. Also possible with Actions, however less user friendly.##
What is wrong with using a filter. A better understanding of the scenario will give you better alternative options.
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As Andrew has said there is not a way to do this, however you can get around it with filters and actions depending how you set your calculations up. Further if you want to help us get multiple value parameter selection, please 'up-vote' it here: https://community.tableau.com/ideas/1313