I have not personally tried that, so I’m not 100% sure. However I think you best bet is going to be using shortcodes to try and do this.
https://bbpress.org/forums/topic/bbpress-20-shortcodes
Don’t understand that completely. Is it possible to make a new comment field and field when you post a topic with shortcodes?
If you want to customize some fields on one but not the other that’s going to require you to write a custom plugin and/or a custom theme. It’s certainly do-able but isn’t going to be done without some research and elbow grease.
You would want to show the fields and then add a conditional that shows/hides based on what page/forum you are viewing.
Is it an easy way to duplicate the bbPress plugin so I can have to of them installed at the same time?
No there is no easy way to duplicate the plugin nor is that recommended at all – it would be nothing short of a nightmare to maintain.
Again, what you want to do is do-able. However you will have to get your hands dirty and do some research or hire a developer who can make the customizations for you.
Ok,thanks for all the help 
Is it easy to add additional fields to the existing bbPress forum? Add more than expample Tags, message, subject? Add example 5 fields extra?
It can certainly be done but requires a moderate level of PHP and WordPress knowledge. It won’t be easy to do if you are not familiar with WordPress and PHP.
Do you know if it is any plugin for this?
No, there is not – at least not at the moment.
You can go through the bbPress plugins on the .org repo https://wordpress.org/extend/plugins/search.php?q=bbpress
