If you have them in the same guild then they are both taxed accordingly to their percentage
To use the above example again.
Player 1 Kills a monster
Player 1 leeches Player 2
Player 1 is part of guild X with 50% tax
Player 2 is part of guild Y with 25% tax
Monster is 100,000 EXP
Player 1 gains 25,000
Guild X gains 25,000
Player 2 gains 37,500
Guild Y gains 12,500