You shouldn't need to use two different color primers. Be sure to wetsand with fine grit sandpaper between primer coats. The purpose of the primer is fill surface imperfections and give the topcoat something to bond to (and to keep a prior paint color from bleeding through).

It shouldn't take very long to sand the old paint off of the cowl. I'd wetsand it under a faucet so the sandpaper won't clog up as often.

Good luck,
Bill DCat