回答
-
It's too heavy to carry home, so tomorrow I'll borrow a motorbike and while I'm at it, I'll buy that.
-
It's too heavy to carry home, so tomorrow when I borrow a motorbike, I'll take the opportunity to buy that.
自分の部屋を掃除するついでにリビングも掃除する。
I'll clean my room, and then clean the living room while I'm at it.