Assigning the task to two people separated by semi-colons should work.  Perhaps what you want is for both people to take action on the task?  In that case, you need to look at the Task Result tab to control how many recipients must respond.  The default is "First Response".

When configuring the recipient in the workflow task, you will need to use the option of "Type/Construct my own" and then map the value from the picker control that contains the semi-colon separated list of users.

this doesn't work - it assigns the task to the string UPN;UPN

i have found the solution, and it is using the 'Split' function

i had to use the 'Split' function to pass in the string of names from the picker, and have the value ';' as the separator