getting 2 hd's to communicate

Is it possible to have two hd's, each with their own OS'es (both Linux), up and running at the same time, and make them communicate with each other; such as having a cron job do backups from one to the other periodically?

At this time, I have one as Primary Master (Mandrake 8.2), and the other as Primary slave (Mandrake 9).
Whichever one is set as Master is the one that boots.
I cannot have one as Primary Master and the other as Secondary Master unless one of them is scsi; mine are both ide.

They will both be set up as http servers (Apache, sendmail, MySQL, etc...), and they will both be identical.
The idea being that if one hd craps out, the other will automatically take over.
If there is a more intelligent way of doing this, I would apreciate suggestions.

I tried, but I'm stumped. I'm a lamer with hardware.

I suggest running 1 os and using a hardware/software raid to do mirroring. Much easier solution then what you want to attempt to do.

Exactly. Running two OSs at the same time is mainly just stupid thinking of the overhead it creates. Use RAID instead, like genlee says. RAID 1 is precisely what you describe here. What it does is mainly that it makes your hard drives appear as one (if you have /dev/hda and /dev/hdc, you can create a RAID /dev/md0 with software RAID), and when you write to that device, the system writes the changes to both hard drives.
In a real fault tolerant system, one hard drive could crash and the system would still be running, but in a normal PC, a hard drive crash would probably make the IDE chipset freak out, so the system would hang, not because of Linux, but because your hardware goes crazy. But you can still reboot and it will work as usual.

I had a feeling there would be a smarter solution than mine.

Thanks for the help (and the insult - wink! wink! nudge, nudge)

Don't mention it! =) (Actually, I just meant it was stupid if you knew the alternatives, but under the GNU General Insult License, you're free to interpret it any way you want.)
Anyway, if you want help on the way, there is an excellent document on software RAID at http://www.tldp.org/HOWTO/Software-RAID-HOWTO.html.

Thanks, that was good info.

Sorry about my delay in replying, but I just spent another weekend re-installing Mandrake 250 freakin' zillion times.
It's a damn good thing I love Linux so much more than windozzzzz.