Security: multiple subscriptions (webspaces) per customer

Hello,

Do I miss something or it's just impossible to create multiple 'subscriptions' for a single customer?
My intention is to allow customer to isolate his/her less secure sites from more secure ones while drawing from single set of allocated resources (i.e. 'hosting plans').
As I understand, webspaces are ment for that exactly, runinning code (and having filepermissions) under different system users.

Thanks!