I don't know that one, but here's a superficially similar joke from somewhere among http://miresperanto.com/humuro.htm :

When the British government invited commercial proposals for the digging of the Channel Tunnel between England and France, one man submitted a bid for only £10,000. “How can you possibly dig under the English Channel for only £10,000?” asked the project manager.

“It’s simple,” replied the low bidder. “My partner takes a spade, goes to France and starts digging. I take another spade and start digging from England. We’ll both keep digging until we meet in the middle.”

“Hm, I see. But what happens if, through a miscalculation, you two do not meet?”

“That’s even better for you!” replied the bidder enthusiastically. “In that case you will have two tunnels!”