I have picasa on win xp and I am considering using Shotwell on Ubuntu and dual booting between systems.
Can both programs access same picture files or will they have conflict with each other?
Short answer. Yes it is possible, providing that the images are on a partition accessible by both operating systems, and not used at the same time.
The problem isn't so much that the two programs won't get along, it's that you may run into file storage issues. Windows of course cannot read Linux file systems, and Shotwell has some known issues working with Windows file systems.
So while the setup may work, it's not recommended.
Another problem: both Picasa and Shotwell work with non-destructive edits that (as far as I know) are not compatible. So if you make edits on a picture (let's say in XP-Picasa) you won't see them when you open the picture in Linux-Shotwell. Unless, of course, you repeat the same edits exactly in Shotwell... (however you manage to do that!)
You can also decide to fix your edits by saving different picture versions, but then you lose a big advantage of both these programs: non-destructive edits applied "on the fly".
Once there was someone trying to import picasa.ini's files (where non-destructive edits are accumulated) into Shotwell database to make the transition much easier, but it did not get far...
More recently, another someone decoded picasa.ini's structure and published the results. Now we only need an import filter to recode picasa's transformations into those of a non-destructive Linux editor (Shotwell or, I would prefer, Darktable)