Is there a way under Windows to open two Renoise at the same time?
Yes, just double click again on Renoise icon on your desktop (or in your start menu, or wherever you keep it) 
When I open two instances the second one can’t use the ASIO driver.
Does the soundcard have multiple outputs? This is generally a limit to ASIO, not Renoise’s fault!! Although if it does have multiple outputs and you only want Stereo from Renoise then make sure to tick the Limit To Stereo In/Out option within preferences, otherwise a single instance of Renoise will try and take all resources.
You will find similar happens if you tried to use a wave editor, such as Soundforge, at the same time as Renoise.
Also that option is not supported by all audiocard ASIO drivers.
Some ASIO drivers only allow one program to access the driver regardless how much outputs or inputs are claimed.
Sucky! Not a limitation I’ve personally come across luckily.
Although it is worth mentioning sometimes you can get more useful behaviour with the ASIO4All driver, which does allow connection from multiple programs, than with the device’s own driver. Especially if it does only have a single stereo outputs and no virtual outputs within the driver itself (many soundcards may only be stereo but allow the mixing of more sources by presenting them as virtual outputs to the applications.)
Well, I know that it’s not a Problem of Renoise 
I think I have to look for what I can do with my soundcards. I got two with NO options for routing and the ASIO4All is not the best for realtime recording. Thanx4help to everybody 
/>