I couldn't find a yo!-gimme-a-hand-with-this mechanic in Macho Women with Guns, 2nd Edition or in the indices of GURPS, 3rd Edition or 4th Edition. (However, given the volume of material available for these latter games, I imagine such a mechanic exists in a cleverly-named a sidebar.)
The Hero System—another 3d6-roll-under system best known as the engine for the superhero game Champions and also an inspiration for GURPS—does have an assistance mechanic. For Hero System, Fifth Edition (2002), was published the Ultimate Skill (2006), and in that book's Complementary Skills (21–22) it says that the GM can allow two or more characters who are in contact with each other (typically in person and who can communicate freely) to pick one of their number—the main character—to make the skill roll proper. However, before the main character makes the skill roll, an assistant who's helping the main character perform the task makes the same skill roll.
For every 2 points by which the assistant succeeds on that skill
roll, the main character gains a +1 to skill.
A third and successive character helping adds a flat +1 to the skill
of the assistant.
(This is simplified for ease of use. Ultimate Skill actually leaves a great deal about helping with skills to the GM, going so far as to recommend how to run things if the PCs are a starship bridge crew with hundreds they can command to help them.)
Macho Women with Guns Example
Alex, who has Macho 14 and who has the skill Do technical stuff +1, is starving and needs to operate her toaster oven. Bambi—Macho 12 but no Do technical stuff skill—is standing right there, and she isn't, like, busy or anything, so she offers to help Alex with the task. Bambi rolls a 7 succeeding by 5 therefore granting Alex a +2 to skill. Alex now needs to roll below a 17—instead of below a 15—to make the Bagel Bites Pizza Snacks.
Note: The game uses the toaster oven as an example of an item requiring the skill Do technical stuff.
In Macho Women there aren't the huge variety of skills available to PCs like there are in GURPS and Hero, though. Combined with the generous default skills rules that see all skills based directly on an attribute that's at least 8 (but usually higher), these rules will likely see at least one other character helping another character with any task. This may create balance issues in a long-term Macho Women with Guns campaign.
Fortunately, Macho Women with Guns is a beer-and-pretzels role-playing game along the lines of Kobolds Ate My Baby!, OG (that's actually pronounced og and not an initialism), and maybe TWERPS (i.e. The World's Easiest Role-Playing System), so balance issues will likely take a backseat to wholesale property damage, explosives, and sticking it to the patriarchy.