Lots of reasons not to.
It's going to cost someone twice as much to remove two layers when it gets redone later.
Not going to lay as flat.
Not going to adhere as well where the glue is. May be because the second layer never gets as hot.
Only reasons I can think of to roof over is to save cost.
04:34PM | 01/18/14
I did that once but would not do it again. Yes, it s cheaper. If you plan to stay in the house, it is best to get the old shingles off to expose the sheathing, See what needs to be done as repairs if anything. New paper/shingles then go on and you are good for 25+ years.