The story mentions a tandem tricycle (which I have never seen, and sounds stupid). It occurred to me I had no idea of etymology, other than it sounded like Latin. AHD doesn't help much.
tandem

SYLLABICATION: tan·dem
PRONUNCIATION: tndm
NOUN: 1. A two-wheeled carriage drawn by horses harnessed one before the other. 2. A team of carriage horses harnessed in single file. 3. A tandem bicycle. 4. An arrangement of two or more persons or objects placed one behind the other: driving horses in tandem.
ADJECTIVE: Having two identical components arranged one behind the other: a tandem axle.
ETYMOLOGY: Latin, at last, at length. See to- in Appendix I.
OTHER FORMS: tandem —ADJECTIVE

The next chapter is entitled VENIT TANDEM FELICITAS.
Which I translate as "Happines comes at last".
So how did we get to tandem = a bicycle built for two?

I said a tandem tricycle sounded stupid. Turns out it is extra stupid. The young man sitting behind controls the steering. Crazy. (Daisy, Daisy.....)

Daisy, Daisy,
Give me your answer do!
I'm half crazy,
All for the love of you!
It won't be a stylish marriage,
I can't afford a carriage
But you'll look sweet upon the seat
Of a bicycle made for two.

We will go 'tandem'
As man and wife,
Daisy, Daisy!
'Peddling' away
Down the road of life,
I and my Daisy Bell!
When the road's dark
We can both despise
P'licemen and 'lamps' as well;
There are 'bright lights’
In the dazzling eyes
Of beautiful Daisy Bell!

Dear Doc

My best guess is a metaphor/literal switch as frequently happens - last=stretched=long=not side by side but one behind the other. Since the key feature of a tandem (a word much misused by journalists in particular) is that the two 'driving' components are one behind the other, though working together (even the horse carriage thingy works the same way), it means that two on a bicycle are accommodated, of course, one behind the other, both (at least the cyclist on the front fondly hopes) pedalling away with equal gusto.

cheer

the sunshine warrior