Imagine there are two rectangular blocks in front of you. Put the two together ___ (end to end? What's the right adverb. The result is two times the length) I wish I could draw here. Can the admin add a clipboard feature?
Put the two together end-to-end.

Put the two together end-on.
In line? Put them so their butt-ends meet? Interesting question...
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 Optilang's reply was promoted to an answer.
Thank you, Optilang. Can I use the similar adverb for people?

For example,

Class, please break into pairs and stand back to back?
Yes people can stand back to back, side by side.
Students: Are you brave enough to let our tutors analyse your pronunciation?
optilang: But can we distinguish between the "twice the length" "twice the height" and "twice the depth" combinations of parallelepipedal blocks?