I'll wash your dishes since I'm already doing them.
I did the dishes you left in the sink since I was already doing mine.
この場合、次のような言い方ができますよ。
ーI'll wash your dishes since I'm already doing them.
「皿洗いついでだからやっておくよ」
ーI did the dishes you left in the sink since I was already doing mine.
「シンクのあなたの食器、ついでだからやっておいたよ」
ご参考まで!