my phone is asleep for the night, but when you're editing the contact info, towards the bottom there should be an option that says "add" i believe. you click it and a menu pops up and you select "another phone number". this isnt exactly what it'll say, but its similar. i have 3 phone numbers for my dad in the same contact. once you click add, it'll go back up towards the phone numbers, you add the next number, and to the left you can click on the buttom and select if the 2nd number is mobile, work, home, etc.
again, i'm going off the top of my head, but hopefully this helps you find it and lets you know that yes, its possible.