Does the resource have to work two full 8 hours days or are you just saying that in a 40 hour time period, the resource can work 16 hours? If the resource can work full days only, then the best work around is to change his/her calendar to only work two specific days. If s/he can work a maximum of 16 hours in a week - you can set his/her max units to 40%. That will spread the 16 hours of work out over the week.