Custom Techniques That Already Exist

[Alexander Trafford]

This is just a matter of using common sense to differentiate basic custom moves and signatures moves.

Yes, anyone with posession could figure out how to posess multiple people at the same time but, like Alura alluded to, there are some custom techniques that obviously cannot be copied without the training of that person (like, i think at least, Goku was taught the Kamehameha and then taught others; or how no one else ever just 'figured out' the Big Bang attack).