Connect two Arduinos using I²C (wiring) Altough having found a lot of documentation concerning wiring of I²C buses, I'm still not sure about the following question:

If both devices use separate power sources, is it a good idea to connect VCC/GND of those two devices?

I have two lines for SCL and SDA, connected to SCL/SDA pins on both devices A and B. The master device's VCC is connected to both lines using a pull-up resistor. Now as far as I understood, VCC/GND of the two devices should not be disconnected but I'm afraid I'd damage something if I'd connect them. As two separate power sources will never provide exactly the same voltage that would result in high currents, won't it? Or should I just connect them using some resistor to avoid high currents but provide the same voltage level?
The VCC lines aren't directly connected - you just have a shared data line with some voltage on it, which is always lower than VCC because of the resistor. Each device "talks" to the I2C bus by briefly connecting those lines to GND, which is common between both devices. Current is limited by the pull-up resistor, so it never gets too high. This makes it safe to directly link devices running at slightly different voltages.