With 2 Guitars, Bass and a Keyboard in the band then the most important thing is to work out who plays in what range and with what sound.
That is something you just have to work out between you, but I am in a similar position as Rhythm Guitar (JTV) and Keyboards in a band that also has Lead Guitar (Les Paul/Marshall + top end backing vocals), Lead Singer (+ Dual Keyboards), Bass (+ MIDI Pedals + mid range backing vocals), oh and the Drummer! So we have 3 vocalists, 2 guitars, 2.5 keyboards (bass pedals for basslines or drones), bass and drums. 5 people.
If the two guitars are playing then you need to be completely different sounds e.g. Les Paul+Marshall v Strat+clean(er) or Acoustic, ideally playing different harmonies or at least different shapes in different positions. If the Keyboards are all going for it then again completely different sounds at different ranges and especially avoiding the mid range as that is where the vocals are going on. And don't forget the most important part than many "musicians" forget - to just shut-up sometimes. If there are 3 part vocals (e.g. June by Spock's Beard) then most of the other instruments drop out leaving just a single instrument like acoustic guitar.
his takes some planning but we really have a lot of flexibility. Listen to bands that do this all the time - like 3 guitar bands such as Iron Maiden or Lynyrd Skynyrd. And like all rules in music sometimes you just need to break the rules for a climatic effect.
This is the right advice. Pre production setup is an important part of rehearsal. You must have tonal separation otherwise you get wash outs where a player turns up but still is cancelled by another. A guitar is a full range instrument especially If you play direct to the PA without a mix Engineer. You might be washing out the others thus they want you to change back.
Playing guitar parts different to each other on the neck is essential for the best seperation and front of house mix especially during instrumentals.
The vocals gets the Lions share of the mids, up to highlows for mosts males,
The rythmn guitar should have a wide mid scoop to envelope the vocals Highlows overlap low mids, scooped up to highmids
The solo guitar should cut thru with a narrow highmid or highlow when with a singer. hihats and cymbols get the highhighs.
This is not set in stone and different genres have their peculiarities.
But to make things simple most records have lots of frequency cuts in them to fit everything into the dynamic and frequency range of the media.
Live there is a wider dynamic and frequency range and more forgiveness but you still have to apply EQ cuts.
Give each player their frequency spaces. You'll sound great and can be trusted thru the PA