2. He is dating another girl on the side. 他同时脚踏两条船.
我想把这句说成 I have my feet in two boats 的人也不在少数吧! 其实正确的说法应该是像这样, He is dating another girl on the side. 或是你可以说 He is dating another girl behind her back. (这个 her 指的是他女朋友的意思)
On the side 这句话你去餐厅用餐时也常用到, 指的是主菜之外的副餐, 比如说 I'd like salad with dressing on the side. 就是说我要沙拉和沙拉酱. 或是 Would you like a baked potato on the side? 要不要再点一份烤马铃薯啊?