By definition, I²C is a bus. Thus, you just tie the SDA lines of all the devices on the bus together, and similarly, tie all the SCL lines of the devices on the bus together. So long as you do not violate the specifications of the bus (e.g., have multiple devices with the same address, exceed the capacitive load limit, etc.), things should work nicely. See The I²C-Bus Specification for the comprehensive list of rules. But, barring aggravating, yet still feasible scenarios (multiple masters, devices with different operating voltages on the bus, etc.), it is relatively straightforward.

--Sasha Jevtic