Auto merge mesh - is there a way?

[SpitForty]

I'm trying to make some airport runways for Aerofly FS2. Its quite easy with a single runway. When two runways overlap is there a way of merging the outer meshes into a single one in a fairly automated way rather than hand tweaking every vertex?

i.e. merge the outer mesh of



with this



I'd be very grateful for any pointers or advice.

Thanks
Phil