Definitely save the bottling bucket for bottling. If you absolutely have to, transfer to your bottling bucket, wash out your carboy and then transfer back to secondary, but you're risking more exposure to O2 that way and raising your chance of contamination. If you only have one carboy, I'd suggest going out and getting a better bottle to go with it. They're fairly cheap in the grand scheme of things. Less than paying for new ingredients if you mess this batch up by racking it back and forth.
"Science + beer = good!"