You will probably find that the impedance is wrong for either the mic or earphones or both, and apart from not working could cause damage to the aircraft radio, or any other headsets plugged in.
When I was learning, once we tried to use adapter cables so the rear passengers could listen, and the first two types we tried did not work, in the end we had to buy the proper splitter which was expensive but it worked first time and worked very well.
So everyone has a valid reason for you not trying this!! Put them back on ebay to get your money back!