There are several related RFCs[0], and the IETF's "Data Tracker" page for the MPTCP[1] working group has a lot of great documents as well.
[0]: https://www.google.com/search?q=multipath+tcp+rfc [1]: http://datatracker.ietf.org/wg/mptcp/
MPTCP creates multiple independent TCP subflows and distributes the data among these subflows. The receiver can then reorder the packets thanks to the additonal data-sequence number space.
It seems like the kernel's just treating it as a "dumb" tcp connection that rx's and tx's on more than one interface. In this case you're taking the problems existing tcp connections have and multiplying them by the number of links. SCTP works around these problems and makes both the connection and the delivery of arbitrary messages more reliable, though seemingly at a heavy cost to performance.