Duplexes? Short Sales? Help?

I am looking at a couple of duplexes...I've walked the properties. Now time to make my calculations, so wondering if there is a different 'formula' to use when considering the numbers and making offers...I've got 2 recently sold very similar comps in same area. Also, one is a short sale.
I know the short sale process can take longer (60-90 days) as opposed to SFH which closes average 30-45 days. And I've got some other info. from my agent to go by. Trying to compare apples with apples...

Anyone who has experience with multi-family props (duplexes)?
Short sales? Procedure? Want to brief me?
Just basic particulars; important things to know, pitfalls, etc.
I would greatly appreciate someone's help.
Thanks in advance!