I wear a "mountain bike" helmet (classified as such b/c it has a visor, I guess!) on my road bike. I like the visor b/c my eyes are very sun sensitive. Even with sunglasses, I never felt like a road bike helmet was enough sun protection for my eyes. Coolness factor may not be as great, but that's not my concern.
You absolutely can wear one helmet for multiple bikes.
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow