This isn't all that uncommon. This kind of innovative approach to creating securitized mortgage pools goes back to the early days of the mortgage bond industry -- when big institutions like Salomon Brothers would create these odd securities called IO/PO bonds. These terms mean "interest only" and "principal only" -- and they referred to unusual collateralized bonds in which the principal payments from one set of loans were bundled together with the interest payments from another group of loans. So a homeowner's mortgage payments -- unbeknownst to him -- may be going to two separate investors who purchased his mortgage from the bank that originally lent him the money.
I'm not an expert in the mortgage bond business, but I believe there are reasons why this type of arrangement works well for certain types of investors. It may have something to do with the inherent "cash-out" risk associated with any mortgage because of the homeowner's right to pre-pay the mortgage at any time without a penalty.