Yes it is possible, tho Steam support aren't always the most helpful about it. The thing is the payment details on file for both accounts must match i.e. the billing address if you've used a credit card must be the same for both, if you have deleted payment details, used cards registered to different addresses and so on then your out of luck.
EDIT: IIRC if you have any games purchased other than via steam downloads you will also need to have to hand a receipt for atleast one of those per account, so at best you will be jumping through hoops and steam really don't seem to like doing it for people.