I am not a webCoRE expert... you would not be able to use devices from different hubs as triggers for pistons without using something like HubConnect. webCoRE Pistons run 100% locally on a Hubitat hub.
Hubitat does have a built-in Maker API app, which implements a RESTful API. So, I am guessing a Piston could make a MakerAPI call to a different hub to interact with a device.
I was using HubConnect with webCoRE with no issues.
I installed on HUB_2 which had LAN and other devices, all Z-wave and Zigbee on Hub_1, and those devices shared to Hub_2.
Worked very well, the only reason I'm no longer using it, is I found Node Red.
With Node Red you can pass devices from both hubs to Node Red via Maker API.