Dropbox modification of file in 2 different places at once

Solution 1:

It depends on your settings. Most dropbox-like programs provide you with a variation of these options:

1. Use the local copy, overwrite the remote ones.
2. Use the remote copy and overwrite the local one.
3. Use the one with the latest modification date.
4. Ask the user (popup) which file to keep.

In the case of dropbox their own website states this

Dropbox won't attempt to merge the changes for you. Therefore, any changes made to the same file after it has been updated will also have their changes synced to all computers, but as a "conflicted copy" of the file. A conflicted file and the original copy Conflicting changes are saved separately

If you want to prevent conflict or even gracefully merge changes in source code, use a source control system, such as CVS or subversion.

Solution 2:

As per https://www.dropbox.com/help/36/en

"If two people or computers change the same file at the same time, the first person to finish uploading their new version will "win." Dropbox won't attempt to merge the changes for you. Therefore, any changes made to the same file after it has been updated will also have their changes synced to all computers, but as a "conflicted copy" of the file. The name of the new file will be the same as the old one but appended with "conflicted copy", the name of the person/computer responsible, and the date that the conflict occurred. This way, everyone's changes are preserved and nobody overwrites another person's hard work. If you find a conflicted file in your shared folder, you may want to discuss which changes to keep with your friend."

So no you won't lose anything but you will have duplicate files

edit: this isn't exactly what you asked as you're not working simultaneously from two locations on the same file but it still explains how dropbox handles conflicts.