For a real challenge but also a great reward try:
JAZZ DUETS Joe Pass Herb Ellis Guitar Book
I'm just editing my response (the above two lines) because I now realize you were asking for exercises. The book I mentioned is a set of transcriptions and not exercises so I take back my suggestion and apologize for not reading the question thoroughly before firing off a response.
However, I am a professional musician and I play in many musical ensembles including duos. I am in fact an expert in the duet format. I can't think of a book right now that will help you, and I hope that further answers here do address your question. I will in fact look out for these answers myself.
In the meantime I can give you some suggestions for evaluating such material or for further research on your part. I am just finishing a book myself that will be published around September, and some of what I am going to say is in this book.
In order to learn duets you need to have a fundamental approach to learning songs in the first place. Here are some basic suggestions:
Learn the root movement of each piece. The root of a chord is the lowest note in the chord and determines the letter name of the chord. In other words, the root of, say, a C6 chord is C. Play just the roots and "hear" this as the fundamental building block of the tune.
Next learn the chords associated with each root. Start with at least two ways to play each chord - for example play a position with the root of the chord on the sixth string, and one with the root of the chord on the fifth string. The more ways you know how to play each chord, the more variety you can add to the duet.
The root movement leads to simple bass lines. After the root movement, bass lines are the second layer in creating an architecture for the tune. You can create simple bass lines by starting at the root of each chord and playing the chord's arpeggio (If you don’t know what an arpeggio is, refer to my final comments). Later you can add scale tones, semitone movement, and so on.
Two things to realize here is that the bass defines the whole tune in a very fundamental way, and that the bottom four strings on the guitar (bottom in pitch… the larger strings) are the same note names as a regular four-string bass, so a guitar literally comes complete with a bass, just a couple of octaves higher. Any good guitar player should be at least a fair bass player.
Next is the melody. The best first piece of advice I can give you for melody, unless you are learning a classical piece, is to learn it in at least two positions, one an octave away from the other. This one simple discipline will pay great dividends in duets or other musical ensembles.
There is so much to say about learning melodies but this is a practical first step (Like the other suggestions I am making here, to go further in any of it is to write a book).
Another thing you should know for playing duets or other ensembles is how to play chord melodies. This is playing basic chord shapes with the melody on top (the highest note of the chord). For this you need to be able to play chords without roots. This is one of the most challenging aspects of playing guitar but it is one of the things that leads to you becoming a professional.
Also know the form of a song. Almost all songs have basic forms like AABA tunes and so on. This is so important, especially when embellishing a song or improvising on it. If you know what I mean by AABA then you understand what I am saying here. If you don’t research this and learn how to define musical form.
To back up a bit here though to basic music and guitar knowledge, in order to play duets well, and to play music well in general, you need to thoroughly know the major scale on the guitar in all positions. If you are a beginner to music, the following will come as a surprise to you. The major scale can be used to see the fundamental patterns of songs, to derive all chords, to define a key, to automatically know six other scales without learning a new pattern and to show you where to use them (modes), and for learning Tertiary Harmony, which, simply put, is the system we use to build chords.
You need to be able to play chords as well as their arpeggios. Arpeggios are chords played one note at a time. But to do this you need to know how to define a chord. For instance, a simple major chord is defined as the first, third and fifth notes from the major scale. We write this as, 1, 3, 5. A minor chord is 1, flat-3, 5. And so on. This is Tertiary Harmony, as I mentioned. To be a good duet player, and a good player in general it really helps to understand Tertiary Harmony. I can't stess this enough.
The other thing you need to know well are basic guitar techniques like picking, strumming, and so on.
In summary learn:
Major scale (all positions)
Picking and strumming techniques
This is a good start.
I hope this can act as a guide for evaluating the literature you find, and also for beginning your own research on these topics. There is a lot more, but these are good starting-point topics to consider when playing with other musicians.
The last thing I will say here about playing duets or any other ensemble is don't just play your part - listen hard to the other player(s). This is where the music becomes magic.