Omnibuses, 2 in 1 books etc.

I've done so with shorts, but not novels. There are a number of authors who have done so. Basic feedback I've read is that it might cannibalize sales of the individual titles. I've found with the shorts that some people still like to read the individual titles one-by-one rather than buy the collection. (Which I find crazy when buying the individual stories costs $20+ dollars and buying the collection costs $5).

Personally, with two books I probably wouldn't do it but would instead see about promo'ing book one with the idea that people will then buy book two if they liked book one enough. You're not going to see 100% read-through from book 1 to book 2 if they're in a series. Some will have bought but not yet read book 1. Some won't have liked book 1 enough to go onto book 2. Some will have liked book 1 but forgotten about you enough to not look for book 2. With my eight-story collection, I found that readthrough dropped each story until the fourth one and then was 100% through to the end with the biggest drop from the first story to the second.

I don't think you'd gain much traction with a two-book combo. It's best to wait until the series is done and then offer a "boxed set" at a discounted price.