" together we stand and divided we fall "
fast songs, with no bass guitar indeed needs a good heavy bass!
slow songs indeed need acentuation, clearity, smoother transitions!
i personally believe in padding along behind the keyboardist, while on the organ! let him dictate the song!......it should be less work for an organist with a capable keyboardist.......your teacher sounds like he is doing just that......you will need to follow his lead...........bass can and will definitley be an issue, when their is a well capable keyboardist around!
focus on the main areas of transitions.........and if playing the melody accompanies his playing, hands down! you should do it!
fact is: when musicians can play music in a way to compliment one another, it makes all of you sound bad!