“Megamorph” changes the original ATK of the monster it is equipped to. Having multiple “Megamorphs” equipped to a monster is the same as having one.
“Megamorph” is always recalculating with every change in Life Points. For example, your “Jirai Gumo” is equipped with “Megamorph” and you have 8000 Life Points compared to your 7000. You declare an attack with “Jirai Gumo” and lose the coin toss, losing 4000 Life Points. Now “Jirai Gumo”’s ATK is doubled before Damage Calculation.
When Life Points are equal the equipped monster’s ATK is unchanged.
You may equip “Megamorph” to your opponent’s monster, but ‘your Life Points’ refers to the controller of “Megamorph”. So if you have more Life Points than your opponent, your opponent’s monster equipped with “Megamorph” has its ATK halved.