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If the list to extend and the list to "append" have the same structure:
MapThread[Join, { {{a1, b1}, {a2, b2}}, {{c1}, {c2}} }]
{{a1, b1, c1}, {a2, b2, c2}}
/@) using a virtual function is one way:
MapThread[Join, { {{a1, b1}, {a2, b2}}, {#}& /@ {c1, c2} }]