There is a page out there that I cannot find, that I did find at one point, that helps you set your default due date to two working days in the future (holidays not accounted for - just Saturdays and Sundays) using XPath only. I know about this post by alec: http://alecpojidaev.wordpress.com/2008/12/30/infopath-codeless-programming-walkthrough-2/
There was a post that would essentially use XPath to look at the current day. I believe this is how the post worked or how I think it should work: Since I want to add two business day from the date created, If created on Thursday, it would be due on Monday (add 5 days to Today). If created on Friday, add 5 days to [Today] to make it due on Tuesday. If Created on Saturday (shouldn't happen but does), then add 4 days to [Today] to make it due on Tuesday. If Created on Sunday, then add 3 days to [Today] to make it due on Tuesday.
It may be more accurate to calculate 48 hours ahead then the eqivalent for 24 hours for each weekend day.