Yes, you can, but it would be a silly thing to do. Currently there is only one type of OtherOS that works. (there is no "linux 5.0", I guess he meant YDL. Get PSUbuntu which is better: www.psubuntu.com)
You can only create one 10gb "otheros" partition under XMB, so what you would do is make a dual-booting partition. Here is how:
1) Reformat the HDD creating the 10gb OtherOS partition.
2) download Ubuntu Gutsy or Hardy. Hardy is best if it's working good, which the site will say. Burn the .iso to a cd or dvd as required.
3) Select System>Install OtherOS and follow the instructions.
You will be asked whether you want to partition the hard drive automatically or manually. Automatic would make the whole thing into one Ubuntu partition, so choose manual. Check what it would do with automatic, though, and write down the size of the secondary(extended and swap) partitions. Set aside half the space for the first OS, then divide it into the primary partition, and extended and swap partitions the same size as they would have been under automatic. You'll probably be making partitions of about 4gb, 256mb and 512mb. Don't use less extended or swap, you need those to make up for ps3's limited system RAM.
Then Ubuntu will install. You better have a usb hard drive or something, because the OS itself will use up almost all of that 4gb.
Now, after Ubuntu installs, you will have an OS on half the OtherOS partition, and nothing on the other half.
4) Go to "Install OtherOS" again, on the XMB, and this time tell the OS to install on the 5gb partition that is still empty.
On pc's, Ubuntu installs a grub bootloader that allows you to choose which OS to choose when there's more than one. On ps3 the bootloader is called kboot. You may have to manually configure kboot to give you a choice, or it may simply be a matter of typing in the appropriate OS at the prompt. Read up on the specifics of kboot to see. I am certain it is possible. It's easiest to follow a trailblazer with linux.
If you don't have a usb hard drive, this whole idea would be really pointless. Also, Ubuntu is great, but you might want to use one ubuntu-size distro and one small one like Puppy or Gentoo. Because 4gb is an awfully small install partition. I don't think YDL would fit on it at all.