1. Is there a way to have two language grammars working on one

language? As an example: There's the language grammar Latex in the Latex-Bundle I have an additional bundle for own snippets and commands. Sometimes I'm coming along a need for having something added to the Latex-language grammar and I would like to Hotfix it w/out breaking it with the next update (or as I just noticed I couldn't even change the grammar). For this I would like to have in my own bundle a Latex-language grammar which includes only the hotfixes and works together with the rest of the language grammar of the Latex-Bundle. Is this possible?

It depends on the language, and what you want to add. Basically the way you can do it is to have a grammar with your rules in it, then at the bottom include the Latex grammar. Being that your rules are higher they will take precedence. However anytime a begin/end rule in the Latex grammar is descended into only it's patterns array is active, so inside that your rules wouldn't be active.

2)What's the best way to edit stuff in the bundle editor? Right now I'm using for longer stuff always "Edit in Textmate" - is there anything else for getting the comfort of Textmate while editing snippets, commands and so on?

That's the recommended way.