Manufacturers can request (or perhaps buy) a largish block of addresses from some standards organisation (I think it's Bellcore/Telcordia/whatever-they're-called-this-week). If they exhaust that block, they can get another, and so on.
Manufacturers are supposed to ensure uniqueness of MAC addresses, but they've been known to screw up (e.g., SGI), so yes, two NICs are supposed to have different MAC addresses. In practice, since we don't run XNS, a MAC address only has to be unique within the Ethernet that its card is plugged into (assuming no BOOTP through gateways). Many cards, not just new ones, permit overriding the default MAC address and the plan 9 drivers generally support this (with ea=XXXXXXXXXXXX in the ether's plan9.ini entry).
