How are people mounting ds10a's on door with door frames? they aren't exactly flush...
On one of my doors I had to add shims underneath the magnet on the door so it would be close enough to be reliable. I just used a couple layers of 2-way cushion tape before screwing the magnet to the door.
Also the front door had double doors that can open, should I do the dual switch setup or what?
You can add another reed switch / magnet in series to the existing one so then you just need the one DS10A to monitor both doors.
Also, is it better to place the contacts closer to the hinge side or closer to the opening side?
Placement is your preference, but you have to keep in mind how much door movement do you want to allow before triggering. The closer to the hinge side, the more the door will open without detection. I prefer mine at the opposite end so detection is made before any open space appears.