I got this working by adding -multi to the command line of OBS.
Actually, when you press right click on OBS exe at desktop for example, go to properties, and add to the target in the add -multi , like this:
"C:\Program Files (x86)\OBS\OBS.exe" -multi
And now you can run multiple instances of OBS, which will give you an ability to stream with OBS to multiple streams at the same time.
Tho, I didn't realise yet how can I stream with multiple instances to the same channel in different resolution, I've tried with my UStream testing account channel by adding to streamkey a code like this _%i_%b_%c but it didn't worked as it works in FMLE tho...
Any ideas how can I get this working?
Thank you! :)