Guitar and piano are both used for chordal accompaniment in a jazz band. So I wanted to find out what's the "best" approach to have this two instruments playing together in a jazz ensemble (quartet) without the two stepping on top of each other?
As a guitarist, if I'm playing with a pianist, I usually do a couple of things.
If I haven't played with them before, I'll usually lay off a bit in order to get a feel for how the pianist wants to approach the comping. I tend to adapt my paying around my bandmates when I'm comping.
If the pianist wants to do some complicated rhythmic stuff, I tend to go for more of a play-the-chords on the downbeat style. I don't think having two compers lay down quick uncoordinated 16th-note rhythms really suits most jazz styles, at least not with a bandmate trying to solo over them.
If the pianist wants to go for some complicated outside substitutions and stuff without really telling me beforehand (and really, they should tell you - common courtesy), I might just play some countermelodic phrases to the solo. That way, I'm not in anyone's way, and I can still add things to the musical soundscape of the band.
On the flip side, I've also played with some pianists who mostly just laid down chill chords within each measure. With that kind of groove, I often find myself starting to play more syncopated comping paterns, filling in more.
General advice: Listening is the single most important skill to have when accompanying a solo, and it's pretty darn useful in basically all musical performance. If you can hear space for yourself within the sound, go ahead and play in that space. If not, you shouldn't barge your way into the song. It's not your turn to solo yet!
There are two things you're responsible when a bandmate's soloing: Rhythm and Harmony. Often, the harmony can be implied with very simple phrases and chords, so I like to keep it simple where I can (especially with a second accompanist). The rhythm is more what I focus on, but you'll likely have a drummer and bassist to help with that.
My philosophy is to keep it simple. No need to bust out your fanciest chops, you're just in the background. The best accompaniment is often the one that the audience never realizes was being played at all! Use good judgement in your comping, and remember that you'll probably get in more trouble for trying to do too much during someone else's solo than for not doing enough.
This is a tough call. Where on the finger board is not really helpful. The bigger issue will be two people comping simultaneously and having the rhythms clash. Do you have a Bass in the group? If not you could always play a sort of walking chord melody bass line in the lower register. That would fill up space and keep the steady groove going and act as a background for the piano to comp.
Is the guitarist the "front man (woman)" like Wes, Kenny, Pat, etc? Or a supporting instrument? I ask because a lot of guitar front men may lay out while others are soloing. Wes, Kenny Burrell, Pat Martino, and others are considered virtuoso guitarists but also play solid rhythm. If you are the guitarist I'd listen some the approach of some of the greats and see how they approach it. Like I said earlier, two or more folks comping will be a disaster. The key is for them to gel and play off each other in a way that is mutually supportive. It has been my experience that guitarists often defer to the piano and let them comp while keeping a steady percussive groove going. In this manner the guitar is acting similar to bass and percussion. This is similar to a classic big band style of playing, and probably not ideal for a small group, but the formula works. Neither player should be over-doing it on the comp as that would kill the mood for the head and the soloist. Depending on the song and its mood/style the guitar can just play chords on the 1 of each measure and let them ring. This works on more etheric tunes. It's about creating a mood.
As for what to play, yes everyone should be playing the same changes/chords. You can try and get creative and have the piano and guitar play complementary poly chords but it may be guilding the lily. This also depends on whether you are arranging scores for the group or just playing out of the Real Book. In the later case everyone will be improvising and comping, you all have no control, so the players need to be self aware and good listeners. They all need to play as a team and not compete with each other. If they are seasoned players this will come naturally. If this is a new venture you all are going to need to feel each other out. If you are arranging scores to follow to the letter, again I'd recommend listening to classic version and stealing ideas.
I play piano myself, and I recently played with a trio with guitar and bass for the first time.
My experience, as a piano player, when having a solo, was that the tight, clustered left hand chords (Bill Evans) worked not so well with the guitar comping underneath. The alterations may clash, especially when the guitarist used tight voicings of his chords. The right hand could work in the normal way.
So I ended up simplifying the left hand both rhythmically AND harmonically, often using only two notes (7th and 3rd) and often laying it down straight on the beat. The thinner voicing allowed for some rhythmic activity as well, and this was welcome since we didn't have a drummer in the band.
When the bass player had soloes, I simplified my left hand even more, frequently playing on the downbeats, with the guitar comping exploiting the off beat chords. And alternating comping from tune to tune also works well.
Everybody need not play all the time. The variety in sound is just another pre in a small combo jazz band, I think.