My father has now two M&M card with ~7-10K miles on each and was wondering how to get all miles on one card

Here is the story:
- In April, my father flew FRA-MEX-FRA and decided to join M&M. He filled in a M&M form at MEX airport and was told he will shortly receive his M&M card
- Six months went by and no card was received
- In October, he flew to Japan with LH

He was really eager to get these miles so I helped him to subscribe online. Since he never received the card from his subscription at MEX, i claimed the miles for these flights. Somehow, this "activated" LH system because within two weeks he received two M&M cards.

Of course, we now have an issue as he has two cards:
- One with the FRA-MEX-FRA miles
- One with the FRA-Japan-FRA miles

This very annoying... He is thinking about sending back on card and ask to credit the miles of that card to the other one. But knowing LH, I am not sure this is as easy.

Has anyone an idea on how to merge these accounts? Is it even possible?