It's called "splitting" a project; and yes, it can be done. If you have your manual, you should be able to locate the directions to do that. There are a number of steps and you must read all the NOTES carefully. If you do not have your manual, you can always go to HELP.
Depending on how you merged your project, you will want to split it the same way (relationship or whatever). You will basically be doing a grouping prior to splitting. It shouldn't be too difficult for you, though if you previously merged. :)
Good luck! Oh ... and make sure after you have them split to check the options of save as separate projects so your place libraries and such won't conflict again.