You have the option you mentioned here:
Zones seems like I would need multiple outputs from one PC.
Local playback from the same PC with 2 zones setup from a single 5.1 sound card configured as 2 stereo outputs. (or 2 sound card devices)
Then run line out stereo cables, speaker cables or a wireless FM type transmitter to another location in the house. Running it from the same sound card pretty much guarantees they'll be in perfect sync.
or
Sync 2 zones together, one from each PC.
This isnt perfect for sync but it usually works well enough across 2 floors.
Here's a quick video of how to link 2 zones using different sound cards on the same PC. It works the same across 2 zones from different PCs however you may experience more drift when the 2nd zone is on another PC.
https://youtu.be/IR_-3-ONXL8or
You have the option of implementing a dedicated syncronized Airplay playback zone using third party software with Airplay. It's detailed in the below thread using TuneBlade as a dedicated server zone with TuneAero installed on both the server and other end points.
This requires more configuration and technical knowledge, more software and Airplay compatible devices or the Airplay compatible TuneAero software installed. BUT it works very well.
http://yabb.jriver.com/interact/index.php?topic=95760.0