yeah it is ... if you do a search on this forum I guess you can find some topics before
anyway, 1st install mandrake on let say /dev/hda5 , set swap at /dev/hda6 then make another partition for red hat at /dev/hda7
after you have finish installing mandrake, reboot your pc and proceed with rh, during this install you can reinstall grub or lilo from this installation and add the mandrake (in /dev/hda5) in your boot options (as long as u remember the perimeters when booting mandrake ) ...
swap can be shared between mandrake and rh ...
p/s: you dont have to follow exactly /dev/hda5 or /dev/hdc6 like above , thats just what used to be my old dual boot rh & slackware box
