Should be straightforward. I've got a triple-boot on AAO with 120GB hard drive.
Procedure in my case: second system I loaded was Linux Mint. During installation, set Linpus partition to 30GB, and programme did the rest, giving a dualboot. Then installed Ubuntu UNR,dividing the rest of HD between Mint and UNR. Installation then automatically set up the triple boot.
There was some command-line work to do to get both Mint and UNR working with wi-fi. Both also work well with T-mobile dongle (Huawei 220) - which was my 'beef' with Linpus.
Now, I use UNR when out and about (fast booty up; economical on battery),and Mint when power point is handy and I want to use GIMP, audio/video and other software. Don't know why I keep Linpus - but it works fine except for above-mentioned problem with dongle...