I’ve come up to a very complicated procedure to be able to get what you describe.
First of all, neither the “1.” not the “Article 1:” is the primary reference in order to get correct list auto numbering.
In the attached AskLODoubleListNbr.odt (19.0 KB), I propose two solutions.
In the first half, I have a “traditional” list with some intermediate unnumbered items. The list uses the Numbering 123 list style I have customized with "Article " prefix.
The number is copied inside a frame (styled MarginNbr). This frame is parameterised so that it lies in the left page border, vertically centered on the paragraph text (modify to your liking). Contents is a cross-reference field to a Numbered paragraph.
As you can see, you can’t get rid of the suffix and prefix defined in the list style.
The list style is the primary numbering source, not the margin frame. I deliberately did it because the inverse would lead to strange numbering sequence: frames are not part of the main flow and are scanned by Writer in a not “consistent” order (something like the timely order of insertion). This results in a non geographic monotonic sequencing. To keep the expected numbering, the list (in the main text flow) must be the primary source.
I tried to be closer your specification in the lower half. It is even trickier than the first solution. Open the file to follow the very concise comments below.
The list is now controlled by Numbering ABC. No prefix nor suffix decorate the “number”. I modified Numbering Symbol character style (used to format the number) to be hidden (but it is ineffective apparently).
The margin echo is done using the same technique as previously except I manually add "Article " before the number because the number is now “undecorated”.
In the text itself, I anchor a ParaNbr-styled frame to the list item. This frame is positioned in the
Left paragraph border, i.e. the indent created by the list style and vertically aligned with paragraph text (so that it looks like being part of the first line). The number is echoed through a cross-reference field to a numbered paragraph. “:” is manually added.
The frame covers the real list item number and its background hides the real number.
When you edit your text, the cross-references update as expected. You still have a little annoyance: if you copy and paste a list item, the frames are not copied with the text. You must manually copy them one by one and perhaps restyle them by applying any frame style, then again the intended frame style.
F9 will fix numbering order.