I live in manchego country and happen to be a hardcore cheddar fan. Here's my proverbial two cents.
If you must pair the two (which I think I would not, but don't let that deter you) with the same wine, I'm thinking like some of the above the following:
Serious sherry (palo cortado and oloroso, especially) and some of the drier types of port (10-y-o tawny is too alcoholic, 20-y-o tawny is indeed very versatile) will sing with both, but I'm guessing you will want suggestions alongside the red-wine-with-cheese school, so:
Aussie shiraz will do the trick for me with cheddar--and simply respect the manchego/red pairing. Better still with the C-9 idea, perhaps, for a potentially (POTENTIALLY I said

) higher acidity. Ditto for some medium-weight/cooler region Aussie cabs.
No Rioja will ruin the pairing, but some old-school, high-acid (Italian-styled) bottles will simply manage to cut through the cheese fat and stop there. I personally would not feel any pressure to pair with Spanish simply because one cheese is so, and in fact under "manchego" you are going to find so many variations that I think my first question should have been
"What Manchego? How old? Ewe's milk only, I assume? Crumbly at all?"
Plus, indeed, do not ignore the white wine suggestions. Actually, it'd be best to offer a choice of extremes; but if we must stick to the original thread name... fortified or Aussie shiraz for me, please.