Is there a term that roughly means that the total is less than the sum of its parts–aside from the obvious jokes?
Dissynergy? Desynergy? Any ideas?
According to my Webster’s, Synergy is combined or cooperative action or force.
So for an antonym: maybe discord, anarchy, chaos, disagreement or something along those lines…
Synergy: the working together of two things (muscles or drugs for example) to produce an effect greater than the sum of their individual effects.
Going to be hard to come up with something that is the opposite of that. How about Paul McCartney & Michael Jackson, singing “Ebony and Ivory.” The effect is definitely lesser than the sum of the parts. You could coin the term “eboivo” to use as the short form. 
Good call, DMC.
If eboivo doesn’t work, I’d recommend either interference, attenuation, enervation, or something similar.
Maybe the OP wanted an antonym for “gestalt”.
What about Antagonism?
Althernatively, if we want to make up a word, I have a suggestion, based on the following logic:
if
Synonym is the opposite of Antonym
then
Synergy is the opposite of Antergy
Pleased with myself for having innovated this word, I typed it into Google and found that I’m not actually the first to think of it.
Damn.
“dysergy” has been used by some writers.
This is interesting:
There’s considerable variation in the definitions, but…
Synergy as we know, is when two desirable or positive* actions have a desirable or positive* effect that is greater than their sum.
Antergy appears to describe when two actions have an effect less than their sum.
Dysergy appears to describe when two undesirable or negative* actions have an undesirable or negative* effect that is greater than their sum.
*(implied, at least by my interpretation, but seldom stated)
Still looking for terms that might describe when two desirable actions unite to have an undesirable effect and vice versa.
The reason I remembered “dysergy” was that Julian May used it in the “Galactic Milieu” trilogy in exactly the sense you were looking for - Marc and Jon’s attempts to combine CE-enhanced metapsychic powers for seismic engineering purposes resulted in “dysergic” combinations (and damn near roasted Marc). Whatever you might think of the plausibility of Julian May’s science fiction, she is generally a good writer, and thoughtful in her choice of words.
It’s not in the dictionary - looking around on google seemed to confirm that the word enjoyed some use, albeit with a variety of finer-grained interpretations. May wasn’t just coining it.
There are already plenty of words to communicate your meaning, Oldmaid.
There’s no need to create a word, other than for the self-ingratiating effect of saying “look how clever I am - I invented a word.” Just because some clever SF author happened to use the word, doesn’t mean that the average reader is going to know what the hell you are saying, and not think “what kind of self-important word-inventor wrote this?”
(Don’t get me wrong, I read some of the Galactic Milieu and liked May’s writing style, the setting, and the premise. The plot just didn’t move fast enough to keep me interested.)
Questions like this are why I like the concept-orinented organization of Roget’s Thesaurus. One looks up synergy in the index, finds the category number it belongs to and then, turning to that category, can use either a short list of antonyms provided there, or the antonmyical category that is usually adjacent.
In addition to Astroboy’s suggestions (I like discord best: “hail Eris” and all that), might I also recommend dissonance, contradiction or simply confusion, depending on the elements that are combining to create a less than expected result. And Cap’n Crude’s suggestion of interference is technically very accurate (specifically, the destructive kind of interference - the kind most people think of when the word interference is used alone).
But you really must give context the most weight when you decide which word to choose. And if you’ve never heard a candidate word used in context before, don’t put it on your list of candidates. It may have a connotation you don’t intend to use. (For instance, dissonance carries a connotation of sound or music.)
If the elements that are combining happen to be persons, then there is a vast addition to the number of antonyms at your disposal - English is full of words to describe what happens when people don’t get along.