Internal LVDS for between-board communicatino

The answer to the typical question "how fast and how much long" is not simple. The signal frequency vs. distance issue is strictly cable-dependant, and especially varies with cable size, quality of the shield and quality of the twist for each pair.

But you are speaking of 40MHz, which is realy low speed for LVDS and you should not have any problem with 20cm. At least be sure of a good shielding of your cable and the resulting eye pattern shoul be clean enough for a correct data recovery.