Anyone familiar w/ Velleman K8064 for diming?

In another thread, I found ppl mentioning the Velleman K8064 kit for diming AC lights. I'd like to use this, but I have a few questions first:

The setup I have now is a bank of four different colored lights for an easel. Currently, they are wired with common ground and using normal dimmer switches for a house (120VAC @ 60Hz).

Could I use multiple K8064 kits to control these and still use common ground (just daisy-chain the neg. on the inputs of the K8064)?

Also, has anyone been able to scale the control down from 0-10Vdc to 0-5Vdc, or should I still expect to have to use an op amp to double it?

Thanks in advance.