First off, everyone expects a certain answer, don’t they? That’s the classic setup of an antijoke. But OP instead seems to be envisioning something more generalized and symbolically representative than a simple
number, such as the union of two *things* (hinted at by the lack of an “=”). Very likely, “1” here refers to a sentient being or – keeping it simple – a person (as in “me and my plus-one”).

Now, an alternate way to refer to such a grouping might be “you, and me both”, or more succinctly “you, and me too”. This is where the double entendre comes in. “Too” here is likely a verbal pun, actually meaning “tout”, French for “all”. Rendering “me and every [one]” – whatever was going on between the two original actors in this scenario, is now going on with everyone. A wild party ensues.

Which brings us to the finale of this finely-crafted antijoke. Whereas most would end with a dry dictionary definiton, historical reference, or mundane scientific factoid (hey, we’ve all done it), this joke instead appiles one more reversal to subvert every expectation. Yet *another* doubled-meaning (final count: 5!) takes us from “tout” to the numerical symbol “2”. The wild party once envisioned has abruptly ended, and we’re left with a stark remnant from grade school math class, a true smack in the reader’s expectant face.

So all that, without a single trick or embellishment – no italics emphasizing a syllable, no apostrophe’d contractions to work around an awkward near-miss rhymes. Just pure, unadulterated wit and idiomatic wordplay.

So 1+1 is 2? yes, on the face of it. But, oh, wasn’t the real journey so much more fun?

P.S. just noticed this one parting gift from the OP: “Tout” read by an untrained English speaker is a homonym for “taut”, which is exactly what this joke turned out to be!