That would be a different Tool altogether. 
NPCs do not receive the special ability to harvest more than one tiny Block at a time like Players do. Otherwise the NPCs would be absurdly OP with their numbers advantage, not to mention Players would not have an edge against NPCs when it comes to productivity (which is needed, or else the NPCs have no reason to hire Players at all).
NPCs might however have multiple designs for the same Tool, like a Doublesided Pickaxe (aka a normal Pickaxe) and a Singlesided Pickaxe (which looks more closely to a scythe but with a point instead of a blade), with the doublesided one being twice as expensive but also half the downtime for re-sharpening, since you got two pointy bits.
Some NPCs might even “design” Multitools specifically to sell them to Players, but you still have to commission them to make those Tools, unless there is a lot of Players frequenting the Area to have at least one instance of the Tool ready for sale. (which is unlikely in Singleplayer). They can still have their “magnum opus” or prototype edition of the Tool laying around though, so even Singleplayer people can purchase from NPCs directly without needing to wait for them to make it.
You could always ofcourse break into the NPCs home and steal their Book to get all the Recipes, but they miiiiiight take issue to that. XD