Selecting multiple entries from a drop down list

Hi, 

Within a spreadsheet i have created a REF TAB where I have created many lists:

• months of the year (jan, feb, mar etc)
• Manager name (jon, bill, fred etc)
• Risk impacts (revenue, lost of service, technical dept etc). 
• .....

Using "Name manager" I have created named listed for each list type.

 

Then on my Sheet1 TAB within certain cells (whole columns) I have used "Data Validation" to create list referencing a list created used "name manager" - This all works perfectly when I only want one entry in the Cell eg the month of the year.

 

my question is - how can I choose more that one entry for the cell from the drop down list eg I would like cell c5  to have 2 "risk impacts" eg the cell shows "Revenue", "Lost of service"