booting to two Linuxes

Hi well that's quite a general question.
depending on the distribution you will be using lilo or grub or whatever boot loader comes with the distribution.
the best thing to do is to go into the config file for the boot loader and create 2 entries pointing to the different hda hdb drives and their respective boot partitions.
a google on the subject should also give you quit a bit of information on how to do it.

1) Partion your hard drive with two partions. One for one distro and the other for the other distro. So if you have a 40 gb divide it into 2 20 gig. So on one 20gb you'll have a / (19ish gig), /boot (150 mb or so...depending on what you want to do), and swap (256 to 512 mb depending on what you are doing) and on the other you'll have a /(13ish gig), /share (7ish gig) (to share your data between distos).

2) Install Distro 1 use Grub as a boot loader

3) Install Distro 2 make sure to update the grub information

4) You now have two distros.