Alright jazzers, and theory buffs. Throw some soloing possibilities my way over these changes from Thelonius Monk's Epistrophy.

C#9 D9 | C#9 D9 | C#9 D9 | C#9 D9 | Eb9 E9 | Eb9 E9 | Eb9 E9| Eb9 E9|

Your Majesty | All Lyrics/Melody/Piano/Guitar parts written by me. Please have a listen.
Well.. the Epistrophy I play is all _7 chords and sometimes the _6 chord functioning as the I.. but anyway.

First thing you notice is that the C#7 chord is followed by a _7 chord a half a step up - doesn't seem like there's any correlation there, but of course there is - D7 is the tritone substitution for the V chord in the key of C#, G#7. So, all you have is a pretty basic I - V - I - V etc.. thing going on because your D7 chord is simply outlining a rootless G#7#11 chord from Lydian dominant. Notes from D7 are D - F# - C which are #11 - b7 - 3 respectively from G# - if you throw the fifth in there, A - we get the b9 of G#, and even further if you throw the ninth, E in there we also get the #5 of G#, Dx enharmonically - the chord then becomes G#7alt but the function is the same.

So with a pretty basic progression like that going on for quite a few bars - soloing over it shouldn't be too difficult - write the notes of the chords down and we get..

C#7 = C# - E# - (G#) - B
D7 or G#7#11 = D - F# - (A) - C.

If you want to play one scale over both - we get C# - D - E# - F# - G# - A - B - C (B#) - which isn't very nice, but there are things that will work.

The first thing that you can see is within those notes the presence of the C# major pentatonic - C# - D# - E# - G# - A# (excluding the A and D, but remember the chord you play does not need to have the fifth or the root - so if you played a rootless and fifthless voicing of say D7#9, you can get away with playing C# major pentatonic) even with your stock D7 chord, C# major pentatonic will still work if you want to just avoid the D over C#7 and A# over D7 - even then, they still work - playing the D over C#7 simply outliness a C#7b9 chord and the A# over D7 simply outlines a D7+ chord. Be careful to try and avoid holding C# (the major 7th of D) over the D7 chord.

Given that C#7 is a dominant chord - it makes sense to also think that C# Mixolydian will work, and it will - but I personally don't like the sound the scale outlines over the D7 chord. In full, if you avoid the C# notes the full chord we get is D13(b9, #9, b5, #5).. a pretty stressed dominant chord with tension all over the place - which is good if you know how to use it and resolve it, but not so good if you don't. It's another option, but not as safe as C# major pentatonic.

Another option is C# whole tone - C# - D# - E# - Fx - Gx - Ax - C#. Which over our C#7 chord gives us the notes: 1 - 2 - 3 - #4 - #5 - b7 (Ax being enharmonic to B, the minor seventh of C#). We're basically outlining a nice C#7#5#11 chord - which is a nice sound to my ears, but it gets even more interesting when we play it over the D7 chord.. (assuming you atleast try to avoid the C#): b9 - #9 - 11 - 5 - 6... another altered dominant chord - D13(b9, #9) - so playing C# whole tone over C#7 - D7 creates two altered dominant chords C#7#5#11 and D13(b9, #9) - that's an interesting way to use a scale to emphasize the inherent tension already in the simple C#7 - D7 movement.

One more scale to think about is D# melodic minor - over the chord we're playing, G# Lydian dominant. Remember me mentioning that our D7 chord is a rootless G#7#11 chord in disguise? Well G#7#11 comes from the fourth mode of D# melodic minor, G# Lydian dominant - so it makes sense to say we can play that over G#7#11, but can we play it over C#7? If we take a closer look - this is what we get.. G# - A# - B# - Cx - D# - E# - F#. In relation to our C#7 chord and the C# major scale we get: 5 - 6 - b9 - 9 - 3 - 4 if we avoid the B# (major seventh of C#) note. That outlines a pretty simple C#13b9 chord (when you have something like that, the voicing of your chord isn't as important, you could play a simple two note 3 - b7 shell voicing, and use the scale to imply all other extensions and tensions to fill out the sound of the chord) - so G# lydian dominant will work over both chords.

The point I'm making here is that you can practically use any scale you want - those are just a few. Don't always look for the most obvious choice of going C# Mixolydian - D Mixolydian - that will get boring, other scales can give you notes that can make your solo sound so much more interesting by adding the tensions I've been talking about.

As for the whole Eb7 - E7 thing - what you obviously notice is that this is the same as the C#7 - D7 movement where one dominant chord is followed by the next a half step up. The V chord in Eb is Bb7, and E7 is the tritone substitution for Bb7 - so everything I talked about over C#7 - D7 can be applied to Eb7 - E7 (Bb7#11) also.

If there's anything you don't understand, or want me to talk about some more options then you know the drill.. but I'll let you chew on this for a while.