ok if I'm getting this right: you want to stripe both disks in one... well thats no big problem... but offcourse you don't want to lose the data you have on there now & preferably keep everything the same but have one big drive instead of two equals...
so this is what you do: first find a friend who has a 60GB or bigger Harddisk (or two smaller ones could work too) copy your system to the external drive (carbon copy clonor should do the trick) & get all your other files onto that disk too (if you have one disk that's big enough carbon copy cloner can completly clone your disk and all the data at once)
thus now your 'old' data & complete install is saved externally
choose the external drive as bootdrive (remember to turn the drive ON before booting or it will not work

)
if this works, reboot again with the OSX disk in your drive pressing 'c'
wich leads us to step 2: boot from the OSX cd/dvd and open Disk Utility
choose the first harddisk and go to the 'RAID' section drag both harddisks to that place wich states: Drag Disks here...
then choose 'stripe' give it a name & press create... sit back and enjoy the ride... now you schould have one 'big' disk inside the mac... if not hmm problem: you can't make a raid with your disks
reboot to the external drive again
run carbon copy cloner and copy everything back on the internal drive (if you can't make the raid, this is allso what you will be doing because the disks will probably be erased (if not... reboot back onto it and everything stays the same as before))
so if everything goes as planned you have a 120GB bootdisk with your original system & everything as before in your mac... so you can choose this drive as bootdisk.
delete all data from the borrowed disk and return it...
Job done
one warning with Striped disks: they save parts of the files on both disks so if one of the disks fails later on... everything will be lost...
have fun
J