This may be useful to tourists who visit Delhi before they go to Agra.
I visited Delhi in Feb 2010 and bought advance entry tickets to the Taj Mahal at Humayun's Tomb in Nizamuddin East. It was not a combined ticket, but it saved us time standing in lines at the Taj Mahal to buy a ticket there. The tickets are valid for a year from the date of purchase.
I asked about Combined entry tickets at the ITDC counter in the airport and Connaught Place, but nobody had any information on them.