Honestly, the balance pot should be at least twice the value of the volume pot and it should have linear taper. Using the wrong pots is probably what's causing your problem. If you want to replace a 500K volume pot with a pair of pots, one for balance and one for volume, you probably should have gotten a 150K volume pot and a 350K balance pot. The volume pot should have logarithmic taper, the balance pot, linear taper.

Wire them like this:
LEFT RIGHT

in 1 ___ in 3 ___
| |
|2 |2
|----- 1 |----- 1
350k | | 150k 350k | | 150k
balance | | 2 volume balance | | 2 volume
| |---- out | |---- out
| | | |
gnd 3 ___|_____|____ 3 gnd gnd 1 ___|_____|____ 3 gnd

If you don't feel comfortable doing the job, maybe you should take it to a shop. They can do this for very little money.