Hello Francois, francoisdlc
Thanks for your question.
If you have a variable product (collar) with variations by size. (For example S - M - L).
We add another attribute: with/without a leash, with values: with/without.
To the variations "with" we put that they use the BOM of the product "dog leash".
In addition, that BOM, you can make as "sellable" so that it can be sold separately.
Each variation would have its own SKU, and the separate dog leash would have its own. You would have to put two attributes on the variable: Size and Leash, both used for variations.
Please, tell me if that could work for you.