Feeding single circuit from two different subpanels (with interlock)
You have to switch the neutral in your application.   Fortunately your garage door load is only 120V, so you can use 2-pole interlocks.
Now, you're overspending on that particular panel.  What you're looking at is a common &quot;QO&quot; 8-space panel ($35) + a QO2DTI interlock ($30) + a pricey 100A breaker you don't need ($65) + a common 30A breaker ($16 because QO) + a socket you don't need.
You can buy just the components and avoid the expense of socket and large breaker: just a &quot;QO&quot; 4-space panel + the QO2DTI interlock and 2 2-pole breakers.  Don't leave room for anything else in the panel; it's an oddball 120V switched-neutral panel.
An even cheaper way to do this is to bring a receptacle fed from each power source... and right next to that, have an inlet.  From the inlet proceed with normal house wiring to the garage door opener (so this section of permanent wiring is isolated from all others).  Then have a short 1' extension cord that you plug into the inlet and move to whichever outlet is relevant.