[sage]

You never really explained why you have two tanks in parallel. With a parallel circuit, you won't be able to guarantee the current will be the same in each tank. Minor differences, even though you have the same parts in each, will result in a current imbalance and therefore unpredictable results in BOTH tanks. About the only way you could run a setup like this is to measure the current in each tank and adjust the current with resistors or something to make them both the same - and also continuously monitor because an on-going changes are going to throw it off too. Too much trouble if you ask me. I suggest you get another supply for the second tank. OR
Why not just do the parts all together in the same tank or in separate runs in the same tank.
Another thought (which I haven't really analyzed so don't flame me guys) try hooking the tanks in series. You'll need a higher voltage - your 30v supply might be enough - but in series at least the currents will be the same and it's the current that is doing the anodizing.
Be aware though that you'd have to calculate the 720 rule on each tank for the same current and different part sizes.

Sage