NO. If you skip the first (or any leg) of a flight, all subsequent legs would be cancelled. If you check the T&C's, they should state that ALL legs must be flown and in the order ticketed.

Thank you I wasn't sure with the flights been different companies

You could always ask nicely, but it's no guarantee.

I did successfully for an Iberia route AMS-MAD-JFK.

I boarded at MAD.

I should probably elaborate since the entire itinerary was openjaw JFK-MAD-LHR and then AMS-MAD-JFK. The ticket was already paid for and I already flew half of the itinerary and found myself in the south of France. I wanted to change plans and go out MAD, instead of backtracking to AMS so I asked the Iberia people if I could.

There are also instances of people asking, being told yes, and then turning up and finding flight cancelled. Unless its in writing. I wouldn't advise.

For the OP, close to zero chance this would work.

Ps nothing to do with it being different companies, in fact with different companies more chance you'd get away with it but it would be a stupid risk to take.

This question comes up a lot and the answer is still no.

Well thanks for all your replies I will pay the extra rather then take any chances.