There are many options to choose. Try Same Srf>Single Dist
Select the edge to extend, this is the edge that falls short on one of the surfaces. Select the edge again for the next pick aa the edge to measure from. Accept the defaults from here on out, entering the distance to extend when asked. You extend varible distances from each vertex if you want. You can also set options like the extend should be tangent to the original. Read these as you pick them.
When they are both extended so that they intersect, do a merge.