No, you can't connect those two batteries in parallel. To connect batteries in parallel, three things must be true:

1. They must be the exact same voltage and capacity.
2. They must be in the exact same state of charge.
3. They must be roughly the same age.

If you connect those two batteries in parallel, the higher-voltage battery will overcharge the lower-voltage battery, causing it to get hot and possibly catch fire.

With the 30A ESC and two 600 motors, you're probably overloading it. Each of those motors is probably drawing 20-25 Amps. A little simple math will tell you that 40-50 is greater than 30...