On the outbound flight, if UA can through-check your luggage then you will not have any problem.
On check-in at TLV you will have a problem as you will be allowed to take 20K per person on an LY flight to FRA.
By the way, the 20K (44 lbs) limit is not only by LY but by any airline on any route that is not to/from or within North America. When planning an itinerary to/from North America it is very important to keep all in one ticket just for the sake of the luggage limits.
And one more thing - the limit is 20K (44 lbs) per person, apart of 23K (50lbs) per piece, 2 pieces allowed on N. America flights (up to 46K or 100 lbs per person).