Yes, you can use Shiseido and SK-II products together in your skincare routine, but it's essential to consider your skin's specific needs and sensitivities. Both brands offer a range of skincare products, and while they may complement each other, it's crucial to patch-test any new products and introduce them gradually to ensure they work well together and there will be no adverse reactions caused by them. Consulting with a dermatologist or skincare professional is also advisable for personalized guidance.