LM317s in parallel connection
Yes, it is possible.  Do not connect the outputs directly together but rather put a small resistor (0.5 Ohm or so) between each output and the summing node.

Check: http://www.ti.com/lit/ds/symlink/lm317.pdf
8.3.11 Adjustable 4-A Regulator Circuit on page 15/16.

You can also look at 9.3.13 which generates higher current with a single LM317, using instead NPN's to deliver the extra current.