A couple of questions, and a stab at an answer.

What's the route?

Have you checked how many seats are left in C? Do you have status?

In other words, what's the likelihood that even if the flight hasn't cleared yet, that it will clear closer to departure.. such as at the 100, 72, or 24 hour window, or even at the gate?

You would need to CHECK IN both tickets and I guess that's your problem.

I understand that you want the miles.. but here's what I'd actually do:

Cancel the economy ticket. Apply the $$$ (minus change fee) to a future trip -- if mileage is key, apply it towards an inexpensive mileage run.

Book the XF award and fly in first.

Assuming it's not an ULTRA cheap economy ticket, you can probably do better by way of mileage for the same money or less.

I'd rather do that than spend the money and the miles in order to fly.

Of course, this is all speculative without a whole lot of info about your specific situation or the flights.

None of this really answers the specific question of whether the airline will allow you to have to reservations on the same flight, check them both in, etc. A new one on me!