Each wall would do its own damage. The damage isn't combined into a single amount.
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A wall of fire isn't an area in grid terms. It's a sheet, which appears on the map as a line.
Two sheets can pass through the same square parallel to each other. If a creature passes through one sheet, then another sheet, clearly it takes damage from both.
If a sheet radiates heat from one side, nothing in the rules says that a wall of fire blocks heat, so it would radiate through into the area adjacent to the other sheet. If both adjacent walls had their hot sides in the same direction, a creature in the affected area would take damage from both.
edit: Jeraa's point is important if fire resistance applies.