If you have ran out of Serial ATA ports on your motherboard, you can pick up a cheap PCI to SATA controller which will provide you with 2, 4 or even 8 more SATA ports.
To be fair, the boot disk was always assigned in the BIOS. Master/slave is just so the controller can address the drives individually (they both get the commands but they ignore the commands meant for the other drive).
If you mean master-slave setups, then no, you can only have 1 sata drive per port. Most motherboards nowadays come with plenty enough ports, mine has 6 (or even 8? can't remember), and I'm currently only using two...
I did not think that was so much a function supported by the SATA controller on the motherboard (or PCI card) but by an additional controller in the hub. Correct?