I have two Electrifly Lipo battery packs, both 1500mAh 11.1V 8C max discharge at 1.5A with a female Deans plug.

Is there a way I can combine the two packs into my electric Lanier ST-40 for power?

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.

I am using a AXI 2826/12 electric motor.

Specs: http://www.hobby-lobby.com/brushless-axi2826.htm
760 Kv (RPM/V)
10-18 Ni, 3-5 Lipo, 12 Ni battery = 14X7 prop
4S lipo battery = 13X8 prop
5S lipo battery = 11x6 prop
40Amp ESC
For 53 – 106 oz planes
Good for .40 - .46 trainers

I am currently using a 13x6.5 prop

Nope, you're not going to be able to do it with those batteries.

First the motor calls for 4S or 5S.

You COULD wire the batteries in series to get the equivalent of a 6S out of the pair, but you'll overdraw the batteries or at very best get very short run times out of them.

Let's do a bit of math.

The motor is rated at 760KV and calls for a 5s lipo using an 11x6 prop.

Ok, so assume 35Amps draw, given that they are calling for a 40A controller...

35A x 18.5v = 647.5 watts or under 1HP of power, which is about right for a .40 - .46 trainer plane.

Your batteries hooked up as 6S would produce...

647.5 / 22v = 29 amps which is just about 20C continuous!

You never want to run the batteries at a maximum capacity, and those are 8C batteries.

You'd burn them up in short order.

If you instead went 3S2P you would get 3000mAh 16C max capacity but now you are down to 11v.

So...

35A x 11v = 400watts.

So now your plane will be underpowered.

Also, in addition to what Opjose said, always consider that with electric planes, your total weight, the watts consumed per pound or ounce of weight, is always figured with everything on board. This includes batteries. So, adding a second battery to the plane would require that you figure the weight of the second battery pack in the overall weight to do your watts consumed calculations for motor/esc/battery pack selections.

CGr.