Okay have done an experiment. I focused the 8 inch Dob on a tree on a mountain about 2 miles away.

I used an 18 mm eyepiece in both situations.

I marked with a felt pen the point on the focuser tube where the image came to focus with the 2 X screwed directly into the 18 mm eyepiece.

Then repeated with the 2 X lens inside the 2 X adapter and 18 mm eyepiece inside that combo.

In the latter, with the 18 mm eyepiece inside the 2 X adapter and 2 X lens inside that combo, the focuser was 14 mm further in than with the 2 X lens screwed directly into the 18 mm eyepiece.

I got a sharp focus in both situations.