I believe that the there may 2 answers to your problem. The first thought would to be to add another power head into tank number2. Of coarse the second power head would need to have the same flow rate as the other one. The only thing that you may have an issue with is if one of the power heads loses it flow rate for some reason like be clogged or any thing. You will have a hot mess you your hands.
The other answer would be to raise the second tank up. If both tanks are sitting on the same flat surface. The power head is going to have a much higher flow rate than the natural flow of the pipe by its self. By raising the second tank you would loss some flow from the power head and gain more flow in the pipe by itself due to gravity.