Correction you get double miles for your spend on the card. So if you buy a 100$ ticket and make LH issue the ticket and charge it to you, you earn 200 miles instead of 100. The miles earned in the flight still stay the same.
LH can issue domestic UA tickets, problem is you won't find an LH ticket office anywhere in the US anymore. LH.com used to allowed UA bookings but that went away about 3 years ago. You can however purchase UA tickets at LH ATOs:
http://www.lufthansa.com/online/port...1096&cid=18002