Is lateral offset in 3.25x10 inch duct allowed?
If I were doing that job, I would contact an HYAC company or if you can find one, a duct fabricator, and have them make what you need. Their offset would be smooth and without the flaws as compared to something you could make in your basement.
Another possibility would be to purchase a piece of 3 1/4&quot; X 14&quot;(this duct comes in 10&quot;, 12&quot;, and 14&quot; widths) and use it in between the 2-10&quot; ducts to produce the offset. It won't be a clean and pretty transition but it can be done. As far as any &quot;code&quot; there may be would be determined by where you live.