Separate reading effort from subject interest
A reader can be interested in a subject even when the text feels tiring alone. That book may still work well when an adult reads aloud or the two readers take turns.
Try a brief sample
Preview a page or two together. Notice whether unfamiliar words interrupt the meaning, whether the layout helps, and whether the reader wants to continue.
The sample is more useful as a conversation than as a test. It can lead to a simple choice: read alone, read together, or save the book for later.
Change the plan when needed
A choice does not need to be permanent. A shared book can become an independent read, and a solo book can become a read-aloud when a section gets difficult.