SE wouldn't do this as I said before they won't risk losing money.
its people pay the extra 1 dollar for the retainers and someone paying the multi character on same server price they are getting 2 free retainers SE is now losing cash.

I doubt SE will do this because it eliminates putting retainers on the store.

they could make it able to trade but make you pay for the free two retainers on alternate account.
this way they lose no money but I betcha you will be like no cause i want it free.

the whole purpose is to try to get out of paying for the 2 other retainers not just to gear up a alt, I believe SE is smarter than that give them more credit.