Hello,

I would go to United Red Carpet Club, as you are both in a premium cabin on UA as well as a 1K. Lufthansa lounge (Sentator) appears to be for Star Alliance Gold and F/C cabin passengers on Lufthansa only (below is a link which may help).

http://www.lufthansa.com/online/port...en&cid=1000390

Enjoy the lounge and C cabin!

Best regards,
Alex