Can two different browser share one cookie?

You can build a cookie-proxy by creating a Flash application and use Shared Objects (SO = Flash cookies) to store data.

Any Browsers with Flash installed could retrieve the informations stored in the SO.

But, it's an ugly workaround.

Just don't share cookies... and find another way to build your website/app.

Every browser maintains it's own cookies. So in general, no this is not possible.

With a lot of hard work you could in theory write an application that sits on the client computer that looks at all the locations the different browsers store cookies, parses the different cookie formats, synchronises them and then writes them out.

That would be error prone and will break as soon as a browser changes how it works with cookies (not to mention that some of the browsers secure their cookies, so you won't be able to get to them in the first place).

In my opinion, this is not practical and I wouldn't even try.