I'm finding that it's not possible to add an additional Hubitat skill to Amazon Alexa.
How can I control devices from multiple hubs with Alexa?
Ahh, nice question! Same for Google Home! I can't!
But you can link hubs and have all devices needed for Alexa in one hub and the skill in that hub.
I was using Hub Link and Link to Hub but changed to HubConnect.
As far as I know, you can't but then again, I didn't try that hard. I simply setup Link to hub / Hub Link to 'mirror' all the physical devices to one hub. That one is running Google/Alexa, etc.
I've since switched to HubConnect because once I got one hub that 'sees' all the devices, I wanted to also sent that to Homebridge, and then I wanted them all in one dashboard (set).
I have three hubs but I did increment slowly so at some point, between hub purchase 2 and 3, I was in the same condition as you.. wanting to have Alexa once again be able to control the devices I migrated to the 2nd hub. The idea of using three hubs was already formed in my mind, but the "ahha" of not being able to have two hubs work with Alexa, and those other 'cloud' apps I had gotten used to, pushed me to switch to three. I originally thought I'd get it working and then convert back to two. That seems pretty unlikely now. 
Yesterday I converted to use the HubConnect compatible version of Homebridge 100% and I can't imagine going back.
