I would like to have two or more developers working on a same app, how can I share my app objects and customization with other developer accounts.
I searched for the answer to this question a while ago and couldn't find anything. I suspect that working in a multi-tenant environment means Salesforce must be extra diligent in preventing cross-domain communication. While it would be helpful for companies that want it, the same scenario would be a nightmare for companies that don't. The latter probably makes up the majority of Salesforce.com users.
One suggested solution I read about consisted of using multiple Developer editions with each developer focusing on their own segment of the app, documenting it as they go, and then sharing their documentation (including lots of screen shots) with the lead developer who duplicates all the individual objects, workflows, rules, formulas, etc. in the master Developer edition.
The documentation then becomes the repository and not necessarily Salesforce. While this would be tedious it sounds doable.
This issue has been discussed online at length in the past by independent developers who didn't want to invest in fully functional business accounts but I never came across anything that looked like a good solution.
Check the Salesforce App Store - never know what solution other may have come up with!