If you want a word that means to place side by side and not vertically, I would stay away from "juxtapose."
I'm looking for a single word that is the equivalent of putting side by side... so the users will remember them easily and won't need to read supplemental documentation figure out what these functions are intended to do
Technically, juxtapose does not mean "to put/place/position side-by-side. It can mean that, but it can also mean to put/place/position vertically--or diagonally, or... But in fact, that is not all.
The definition in the OED is rather disappointing:
To place (two or more things) side by side, or close to one another, or (one thing) by the side of another.
Oxford English Dictionary, aka OED
I mean, yes, the definition includes side by side, but that is not intregal to the definition. And note that the OED does say it can be other than side-by-side.
So what the function of juxatpose meant would confuse me, because it means..., actually the definition in the little Oxford Dictionary online (ODO) is better:
Place or deal with close together for contrasting effect: black-and-white 'photos of slums were starkly juxtaposed with colour images'
Notice: 'for contrasting effect'. This is integral to the definition, or at least to how the word is used nowadays.
Back to the ODE, two of its three example uses show this:
1879 Cassell's Techn. Educator iii. 191/2 When colours are juxtaposed, they become influenced as to their hue.
1881 H. Spencer Princ. Psychol. (ed. 3) I. ii. ii. 171 They are juxtaposed and contrasted.
Notice the colors could be juxtaposed vertically.
The OED definition provides example uses over 100 years old, but today the word juxtapose primarily means to place close together (not: side by side) for contrasting effect.
Note the entry for the synonyms in US Thesaurus (ODO):
Synonyms of juxtapose in English:
1 the exhibit juxtaposes works by Van Gogh and Gauguin
place side by side, set side by side, collocate, mix;
Note that the works of Van Gogh and Gauguin are not "placed side by side" because that is kinda, sorta alphabetical order, but because of the contrast between the works of the twp artists.
Note these images for "juxtapose":
The glass and the beer bottle are not simply next to each other, they are in juxtaposition:
The fact of two things being seen or placed close together with contrasting effect: the juxtaposition of these two images
Lining up can mean side-by-side, but it can also mean behind one another.