Depends on your application, and the timing requirements.
And your sensors, of course. Some have a fixed address, or one or two address pins, so the number of identical sensors (ICs) on one bus is limited. And you will have a hard time getting standard speed devices to work in high speed modes.
Two busses can reduce the update cycle time to about half. Code size increases minimal, assuming identical peripheral blocks. Two additional connections should not have a large impact on the PCB layout.
