I'm not  sure about the sub account...

But yeah, you could technically create one account, share it and get ps+. Both of you would get the full benefits of ps+. As though whether is legally allowed... I'm not sure...

But who's going to find out?

You are able to share 2 PS3s and 2 PSVs on 1 account with / without PS+. It's legal.

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Added on November 22, 2012, 4:49 am
u can share ps+ account on ps3, example you are A and your friend is B. you got A account, your friend got B account and you both create another PS+ C account~
you can login C account on both you and your friend ps3 to purchase game and download onto the ps3, after that login your own A and B account to play the game~
this doesnt work for vita cause vita can only use 1 account per console, unless you use C account to buy game and download onto ur fren ps3 and use C account on ur vita as well~ and 1 game cant b share for more than 2 same console, which mean you only get to download the game onto 2 vita(you have to play the vita using same account) and 2 ps3~