I'v tried that, it doesnt work too well.
I punctured after 20 miles in a 22 mile time trial. It was a 2up as well, so i didnt really want to stop. I rode on the flat for a bit but i had to stop, im fairly sure it would have damaged the rim if i didnt. Tubular rims are not flat and the edges stick out similar to (but not as much) as clinchers and they take some fairly hard hits with no air in the tyre.
Mine are carbon rims, but i think you could probably ride on alloy tubs because even if you did dent the edges of the rim, it wouldnt have the same problems as a clincher where the outer edge holds the tyre in place.