9v Batteries in Parallel?
You can parallel these batteries, but there will be some mismatch and they won't share the load equally.

A much better solution is to use a buck switcher instead of the 7805.  As it is now, your power supply is only 56% efficient.  You should be able to get that to about 90%.  That efficiency boost alone may be enough to not need a second battery.

If you can use multiple 9 V batteries, you can use different batteries altogether.  9 V batteries have poor power density.  With a small boost switcher, you should be able to run your device from two AA batteries for much longer than the single 9 V battery the way you are using it now.