、。
Fine, I'll let you double up with me, so you pedal — I'll ride on the back.
Literal
Well, two-person-riding [obj-を] allow-[do-for-you-てやる] [because-から] you pedal, I [subject-が] cargo-rack [on-に] ride [because-から].
２人乗り refers to doubling up on a bicycle, technically illegal in Japan. 許してやる uses ~てやる as the condescending/casual giving auxiliary ('I'll let you'). 漕げ is the plain imperative of 漕ぐ ('to pedal, to row'), blunt and masculine. 荷台 ('cargo rack') is the flat rack behind a bicycle seat. The sentence ends with から giving the reason — 'since I'll ride on the rack' — left hanging as an implicit 'so you do the pedaling.'