Another option is to split it into the two separate books.
If I was doing it for my library I'd probably split it. (I say that, but lately I often can't be bothered to do so anymore)
Or otherwise number it 1.2 and if I added the
second volume the publisher offers number that 3.4, although I find that numbering system less than ideal since more and more series are having multiple shorts or novellas between books that might use those numbers (in a general sense, not this series in particular). Maybe 1.02 (& 3.04) instead of 1.2 of course either way this becomes a problem for series with a single novella between books where that would normally be numbered 1.5, etc.