I don't understand.
Fine.
As far as I know I'm just trying to reduce cross-talk on the I2C line between the devices.
There's three parameters for digital coms that need be considered.
Impedance of supply.
Decoupling.
Crosstalk.
Each can be addressed separately and sometime one remedy will be a fix.
Crosstalk is less common and can be mitigated with pull up or pull down resistors on the data lines.
It requires all that because the power supply is 15' away?
Yep.
Some time scoping the data and VCC lines with various solutions to the above will show you that these things
sometimes have little effect on the operation and other times show that there are indeed effects that might of might not effect proper operation.
This is the difference between throwing something together and it usually working or employing decent design practices to ensure it works and under any condition.
Experience gives us clues to what might be needed to make a circuit bulletproof beyond just the basic active components that make up the guts of the circuit.
When you deal with digital devices, the fast risetimes from their outputs suck little chunks of power and these need be managed and provided for in the design. The supply need be low impedance and local bulk capacitance provides for this. Decoupling caps manage the amount of impact the draw of the chunks affect the supply and maybe also affect another device on the same supply.
When a prototype is complete it is very important to verify the VCC rails are not influenced too greatly by normal operation so that communication thresholds are not challenged by blips on the supply rails.
Some time on a circuit with a scope will reveal all these
little things.
If the device has a USB interface, you must not exceed the 10uF limit on bulk decoupling directly on USB Vbus, or it wont pass the USB inrush current test.
The USB source type is not disclosed.