homework and exercises - Inductance of two parallel wires

I've been asked to calculate the inductance per unit length for two wires or radius $a$ separated by $2d$ where $2d>>a$.

Starting from $\int_{s} B.dS = LI$ Im not sure what surface to take?

For each wire the field at a distance r away is given by $\int_{l} B.dl = \mu_0 I$ and by superposition $B$ in the first integral is their sum. And so $$LI= \int_{s} B_1.dS + \int_{s} B_2.dS$$ where $B_1$ and $B_2$ are the contributions from the two wires.

Answer

Answer if anyone is interested. In the end the areas outside the inner edges of the wire cancelled by symmetry and so the surface i was looking for was the area enclosed.