Originally Posted by MichielR

Call LH and have them split the PNR first. Once you have your own PNR you can do whatever you want without effect on the other. I have done this before, albeit not with cheapo Y bucket tickets.

I would then cancel and ask for a refund of taxes of your single ticket, but if you would risk these by being a no-show, or would rahter go to the lounge, that is up to you of course.