You haven't given enough information to determine exactly what you should do, but:
1. It sounds like you want to "dual-boot"
2. The key your "boot loader", which allows you to select one or the other operating systems.
3. Here is a good, brief tutorial on one of the most popular boot loaders, "grub":
4. Your first step is to familiarize yourself with grub, and some of the startup options.
5. Next, you need to determine whether or not you still *have* two operating systems (or whether you inadvertantly removed one OS when you installed the other.
You can easily check this out using grub.
6. If necessary, you'll have to re-install the second OS. This can easily be done in a way that lets you keep the first one. Google, or post another question to LQ in a new thread.
'Hope that helps .. PSM