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That sounds like a fun little project. Yeah, the sunset colors will be gone before the stars appear. What you are asking is actually pretty easy and would just require a pretty simple layer mask. It just depends on how high the Saguaro is in the scene as to how difficult it might be to blend, but if you are letting it silhouette, then that will help too.
So, if possible once you find your sagauro cactus and get your sunset shot, ideally, you would leave the camera in tact in that spot, don't move or touch a thing, especially the focus, as getting the shot focused at night is actually one of the hardest parts, and where those new to shooting stars mess up the most in my experience.
Okay, so you shoot the sunset... don't move the camera, what I would do then is after the sunset fades, keep shooting occassionally, especially as twilight fades.. As I am thinking this through (and it depends on what cloud coverage you have) you would want to blend above the color of the sunset, where the sky is turning blue again, and then with the stars. The closer you shoot the stars towards sunset, rather then say at midnight, the WB of your camera will more easily keep the sky a blue... which will make it easier to blend...
For your star shots, not sure what experience you have with that. If you just keep taking the occassional shot after the sunset you will keep adjusting your exposure. First key really, well you can try it different ways, but if you want your stars still, don't let your shutter exceed 30 seconds. So as the sun fades, I would keep increasing the length of the exposure to about 20 seconds or so, at that point I would start opening up the fstop until you got down to about f8, at that point then start increasing the ISO until you get to about ISO 1600. Don't trust your LCD anymore, the shots will appear okay but will be too dark as the LCD in the dark will make underexposed shots look okay. For stars, make sure your histogram is showing about 2 stops underexposed, that will work. Again, trust the histogram, not the LCD screen at this point.
So depending on how long you will be staying to shoot the stars, when the sky is pretty dark, my ending settings on shooting the stars is usually around f4, ISO 3200 and a 30 sec shutter. For fun... when you are done, before you move the camera... try a star trail shot or two... You can drop the ISO to ISO 400, go to f5.6 and then do a 5 minute exposure, and then maybe try a 10 minute if you are adventurous... but that's up to you. The simplest is the sharp stars with keeping the exposure under 30 secs like I described.
Now back at the ranch... Convert your raw images. Then open up both images in PS.
1. While Holding down the shift key, drag the Sunset shot on top of the star shot. (Holding the shift key will auto center the layer on top of the background)
2. Use the Lasso around select the top of the sky down just into the sunset area.
(Make sure you are on the top layer)
3. Go to the Select Menu, choose Refine Edge.
4. Adjust the Feathering. You might have to try this step once or twice to get the right feather amount, but start at 250 for a feather amount.
5. Hold down the ALT key and click on the Make Layer icon in the Layers adjust panel. It will be a small rectangle with a circle in the middle.
6. This will automatically generate a layer mask from your Lasso selection, allowing the star part of the sky to come through.
And there you go... that's it basically... you might after seeing the look, decided what you selected with the lasso should be more or less... so just undo the couple of last steps and reselect, etc... if the Saguaro is pointing up high into your photo, you can go back with the paint brush and with the color white, click on your Layer Mask screen in the Layers Adjust window, and then paint some of it away, etc...
I hope that helps... And I am pretty sure you have a good handle on PS from seeing your other work, but I still wanted to go through the PS steps with the Layer Mask just in case.