You can say: 'If you buy two juices, you can get a 50% discount on the second cup' or, more naturally, perhaps 'Buy one juice, get a second one half price.' By the way, we don't generally drink juice from cups. Cups are usually for hot drinks like tea and coffee. Juice comes in glasses.

I think we would say in English - "Buy one and get the second one half price." - as we usually expect people to buy two.

第二杯半价：Half price on second one.