This is the 2897th most frequent German word.
zweier
"Zweier" refers to a pair, duo, or two-person team in English.
Here, 'zweier' is used as an adjective referring to the 'bicycle for two persons'.
Ich kaufe einen Schraubenschlüssel für den zweier Fahrrad.
I am buying a wrench for the tandem bicycle.
In this sentence, 'zweier' is the genitive case of 'zwei', referring to 'of two towns'.
Er bringt eine Karte des Flusses und des zweier Städte mit.
He brings a map of the river and of the two towns.
Here, 'zweier' refers to the card with the value of two in the game context.