Using the same iTunes library with two different versions of iTunes
Sharing a physical library between iTunes instances won't work due to various reasons (e.g. incompatible library formats, conflicting access) but you won't need to do that. Home Sharing in included iTunes for exactly this reason and allows you to access the library of Mac #1 from Mac #2 (or vice versa, depending on where your library is actually stored).

There are actually two options here:


Open iTunes Preferences on the Mac were the library is stored, go to Sharing and make your library available to the local network. You should now see the library in the sidebar of the other Mac
Create an Apple ID if you don't have one already and use it to enable Home Sharing via File > Devices > "Turn On Home Sharing" on all involved computers. The benefit of this approach is that it also works with iOS devices and Apple TV. For details see Understanding home sharing and Setting up Home Sharing on your computer.