Create 2 or more separate contacts, one for each mobile number, with the same name if you want (but doesn't have to be). Then link them. You will then see multiple options to call and text.
But that means I'll have either a duplicate Google or Windows Live contact for that person. Which will also be synced back to my Google/Live online accounts. Hardly seems like a neat way of managing contacts. -.-
I just leave the other contact unlinked but give it a descriptive name like "Tony Blackberry" versus "Tony McPherson" so I know which number it's going to. If they're linked it basically takes the main contact name and attaches the new contact to it, which gets confusing for me since we don't bother remembering numbers anymore.