Is there any reason the open source world requires two privacy enhanced browsers, i.e. Tor Browser and JonDoFox?

Apart from the proxy backend Tor or JonDo, they share pretty similar goals. It were good if you two could work close together and review the other ones work.

Here is the idea:
1. Try to merge as much security/privacy/anonymity features into Tor Browser.
2. When 1. is done and you are not happy with the result, fork Tor Browser and add your tweaks.

You could merge so much code. Even the JonDo proxy specific thing. In the final stage you would simply have to add one pref to make TorButton look like JonDo and to use JonDo proxy settings instant of Tor proxy settings.

Wouldn't it be a good thing if all proxy users would share the same fingerprint, i.e. operating system, fonts, etc. (Panopticlick stuff)? Some people like to switch from Tor to JonDo an back, like you support with your button. Wouldn't it be good if they have always the same fingerprint as all Tor users?

No.
We are already working close together although that might not be visible.
Yes, that is the plan. We are working on a JonDoBrowser which includes patches the TorBrowser already has and which is going to launch as an alpha version hopefully next month.
Yes, having just one fork to maintain and sharing a testing environment etc. is desirable. And there is work going on in this area as well. Stay tuned.
Definitely.