AFAIK, you can only use your US miles to upgrade US flights, and only to business class (not that US offers trans-Atlantic FC anymore). You need 60,000 miles to upgrade from an eligible fare on a r/t basis, or 30,000 each way. So only one of you can upgrade in one direction only. Most US international upgrade coupons arenot transferable and may require status to use. Not sure if any LH ones exist. You may want to try posting in the US and LH forums.