You can wire the two packs in parallel so you'd maintain the same voltage but the total capacity would be the sum of both batteries. You can make your own harness to achieve this or I believe they are available pre-made.

What kind of motor is in the plane? Will 3000mAh be sufficient?

I'm definitely no electric expert but with 8C batteries you probably want to make sure the application isn't very demanding, you select the right prop, etc.