Based on the embassy review here http://www.visajourney.com/reviews/index.php?trim=no&cty=&cnty=Malaysia&page=1&dfilter=5&topic=K1 Visa it looks like the US petitioner can attend the interview. Note that you'll be entering via separate lanes at the guardhouse but you can be seated together when you're inside. The US petitioner can also go up to the interview window together with the alien beneficiary but questions will be directed to the alien beneficiary only.