I have a D1 studio kit, is it better to add another D1 or is the D2 also compatible?

The D2 can be fully integrated into a D1 system. You can use TTL on the D2 and still use the D1 in manual mode together. A typical application could be the D1 as background lights and the D2 as main light in TTL. The model could then move around in front of the main light and the exposure will be correct on both the subject as well as on the background. HSS on the D2 unfortunately can not be used together with the D1.