I think we need to understand what type of music you play. Many years ago, I went from two guitars, bass, drums and keyboards to one guitar, bass and drums and we were worried that our stadium rock sound would sound empty. It was the total opposite and the space was a wonderful addition to our sound and we had amazing seperation between the instruments. We went on to play Brixton Academy and many festivals and we had no issue creating a wide full sound and we never missed the other instruments.
These days I play in a 3 piece which is a singer songwriter on acoustic guitar, drummer with brushes and myself on fretless bass. We play with so much space and dynamics that you have to be creative in you area, otherwise it's going to sound flat and repetitive. So to fill the sound out in certain parts, I will add an octave which just fattens the sounds and the next level up after that is to introduce a little envelope filter to the octave sound. Other times I'll use a little chorus and reverb to make a beautiful sweet tone and then some time it will just be a clean tone.
So it really depends on what type of music you play and to decide on respecting the space or filling them but always try and maintain seperation between the instruments.