Distance between UART and I2C bus
Of course multiple signals on a cable always affect each other.You might rather want to know how this effect can be reduced and how long you can expand your cable at a maximum. 

If you seperate these signals, you would have to provide each cable with a seperate GND wire. This can lead to another kind of problem: You will have a big GND loop where external noise can be coupled in. 

If you want to use a single cable, you have least interference when you reduce the inductive loop areas between all signals. This could be done with a layout like this: TX / RX / GND / SDA / SCL / +5V. (GND should be in the middle of the cable and UART and I2C should be seperated).

I2C is not made to be extended because it has tight requirements for the capacitive load of the bus. Maximum extension length depends on the chosen speed of the I2C bus. For a rule of thumb, take maximum 1 meter cable length for 100 kBit speed, and 10 meter cable length for 10 kBit speed.

UART maximum cable length also depends on the UART speed and on the implementation (RS-232, RS-485, TTL,...). You must specify which type you are using.