It did.. anyways here we go again, and do say it didn't work because i tested it on crux
#showtooltip Shiv
/equipslot [mod:alt] 17 Retainer's Blade <------ Inventory weapon
/equipslot [mod:alt] 17 Merciless Gladiator's Quickblade <------ Default OH weapon
/cast [nomod] Shiv
Hold alt to exchange weaps. When not pressing any modifier it just shivs.
Enjoy
Edit: To clarify: You push this once with alt pressed to change to the inventory weapon, push it again without alt pressed to shiv, and then again with alt pressed to switch back. I know in that it can be done with one activation in world of luthland, but in WoW it can't