I cannot speak for the OP but i was looking to install XAMPP along side a copy of parallels plesk because i need an isolated testing environment for my web application along with some functions like curl and system exec i do not want available on my plesk setup.
The problem i faced is that the plesk installation uses the same service names as Xampp does for apache and mysql which should be the same problem experienced with installing multiple instances of Xampp.
I have figured out a dirty work around which might be useful to others. Just change all the xampp service(s) ports (search forums if you do not know how to do this) and then instead of installing the services get a copy of FireDameon or if money/piracy is an issue use Scheduled Tasks to run the various start bats in the xampp directory on system startup. Its not a great solution but so far its worked for me with the only drawback being it kills allot of functionality of the xampp control panel.
In most situations you can make one installation of XAMPP do everything you want just by using Vhosts/careful configuration of the services and this should be considered a last resort. The only reason i did it is to keep it easy to remove after i am finished and avoid any possible security repercussions from allowing the extra functionality. If you do use my method i think it would be interesting to post why you needed it here