Rebel,
Some people do fine training bodyparts twice a week and are able to recover and progress. They are NOT the majority. Most people need more time between workouts. In the end, all that matters is what works and if you can bench on Monday and then dumbell bench a few days latter and make progress, by all means do it. But.........if you are not increasing your weights (or reps) by at least a little each workout, you are WASTING YOUR TIME. Until you are very advanced weight increases should be very consistent. So many people go to the gym week in week out and basically keep doing the same thing week in week out, ad although they may regulary change the lifts they do, they rarely go up in weights, and they wonder why they don't grow.
Iron Addict