You've done the most important optimization available by placing the database files (the iTunes library files) on the local drive. If you had that on slow storage (and Airport Extreme is in the slow category as far as network storage) then I could see you having slow launch times.
Even a library with 70,000 items stored in it should only have an iTunes Music Library.xml file that takes about 120 MB of space which should load in less than 10 seconds unless your PC is really bogged down with other tasks on the CPU.
The actual size of all the media doesn't affect the application launch time as it only has to parse the library files and not check each and every file before you can start using the program.
I would look at tuning up the PC or speeding up the filesystem where the library files are and see if that results in better app performance.
iTunes keeps a lock file in the directory to see if there is an instance of iTunes running - only one is allowed at a time.
Similarly, if you're using the same directory, each version of iTunes will overwrite the library file, so trying to maintain two different sets of music within one tree isn't really possible. At least, that's what happened last time I tried this.
You're better off having Home Sharing enabled, and then having one Macbook at a time run iTunes pointing to the library.
The other can then use a blank dummy library, and play whatever the first is sharing through Home Sharing. Either can run in each role, just hold down Option at start-up to decide if you want to use.
Personally, given a family with a bunch of iPhones, AppleTVs, and laptops, I have a single iTunes server machine that is always on, with a very large library. All music is in my iTunes Match account, all videos can be synced over the air through home sharing, and wireless syncing. It's the closest I've found to an iTunes server device.
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