Crafting an item with multiple skills only levels primary

Engineering is 17, Blacksmithing is 9.
Item was a Spear(9) with the Scorcher augment.
Blacksmithing raised by about 1/10th of a bar, engineering remained unchanged.
That might explain it then, since the engineering is significantly higher than the augment, which is gained relatively early.

If I understood your post correctly, crossbows are still likely to be the only viable means of leveling engineering as a primary craft, since they are the only dedicated engineering craft-able so far. Any other item would level the lower skill much more greatly than engineering ('steal' the skill XP, so to speak).