Bandwidth Management: About HTB

inner queue: queue that has at least one child
leaf queue: queue without children

at first limit-at (CIR) of the all queues will be satisfied, only then child queues will try to borrow the necessary data rate from their parents in order to reach their max-limit (MIR)

SUM of committed rates of ALL children must be less or equal to amount of traffic that is available to parent
Maximal rate of ANY child must be less or equal to maximal rate of the parent

Priority is responsible for "distribution of remaining parent queues traffic to child queues" so that they are able to reach max-limit
After satisfying all limit-ats HTB will give throughput to queue with highest priority

HTB works in one direction and is implemented on outbound interface

Source:
Manual:HTB
http://wiki.mikrotik.com/wiki/Manual:HTB

Question:
It says that "HTB for download will be on ether2 and HTB for upload will be on ether1". How do we do this since in real world there is only one ethernet for download AND upload ?