I used to use Komplete Audio 6 as my audio interface.
Recently bought Yamaha MG18XU mixer which is having USB interface.
What would the setup for these two as audio interface simultaneously. should I have to use ASIO4All or currently Sonar ASIO will work?
 
Regards
S Ghose 

You can't use ASIO as it only allows one driver at a time. Not a Sonar limitation but ASIO. Would not recommend you even have asio4all installed as it can interfere with other drivers.

Your best bet would be WDM but two devices usually don't play nice together because there's no way to synchronise them u less you have a unified driver with devices from the same manufacturer.

So you're out of luck really. What is it you're trying to do. Maybe there's a work around.