Imported lists can only have one one column for email addresses, as NetCommunity will map to only one column for the email address to send to. 

We recommend to create extra rows for the additional email addresses.

For example:

NameEmail Address
Lois Laneloislane@test.com
Lois Lanesuperman1@test.com

For more imported list considerations, refer to How to create an imported list.