You cannot boot an operating system from a slave disk. However, there are a number of possibilities:
1/ Use loadlin. This is a DOS based program to load a linux kernel from a booted DOS (and possibly also Windows) machine.
2/ Configure the second harddisk as secondary master and use lilo (or another boot manager) on the MBR of your first harddisk.
3/ Create a small /boot partition at the beginning of your first HDD (30M is sufficient). For this solution you would also need lilo.