Do you have a home network? You can set up itunes Home Sharing and transfer your purchases that way. You need to authorize each PC for the two different STORE accounts to transfer each account's purchases.
If you don't have a home network you'd have to use an external hard drive or USB stick.
Long story short, there's no automatic way to do it. You need to keep track of each account's purchases and what PC they are on, then move them to the PC that is missing those items.
There are some 3rd-party programs like Tune Ranger that say they will keep everything insync from one PC to the next, but they cost around $20 or $30.