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Is it possible for myself to select where photos overlap eachother?
Synth does not always recognise where photos overlap!
Is it possible to add photos to synths I made earlyer and select where they overlap?
If I understand you correctly you want to know if you can manually arrange photos that did not match each other. The answer is no, it is completely automatic, so the solution is to learn how to take (possibly more) photos so that Photosynth can't help but see how things overlap.
Think of it like this: each time you make a new synth, Photosynth is "seeing" things for the first time. It has never seen a picture frame or a wall or a table or furniture before in its life. It needs to look at things from as many sides as possible if it is going to "understand" their shape. It can't be just shots of things from the top, bottom, front, back, left, and right, either. It needs photos in between those positions so it can see how one side connects to another.
Think of it as if you had been blind your whole life and just had surgery so that you could see for the first time, then think of each photo as blinking your eyes open. How often would you open your eyes to understand?
Your second question: Can you add more photos to synths that you made earlier?
Answer: Not in the sense of coming to the site and adding new photos to an existing synth, no. If you still have the photos from the original synth on your computer, you can make a new synth with the old photos plus new photos to fill in the gaps.
If you do this, the 'calculation' part of making the new synth will take as long as normal, but it will figure out which photos you've already uploaded to the site and only spend time uploading the new photos so the upload should be a good bit shorter than normal, depending on the number of new photos that you're including in the new synth.