Hi, I will travel from HKG to Shenzhen and Guangzhou and back to HKG for another 3 nights stay.

Well, Standard Octopus Cards cannot be used in Shenzhen. So, you are advised to get Hu Tong Xing, which is applicable in both HK and Shenzhen.

You need to pay HKD 98 for it, and add some value before use. The card carries payment function in respect of the Octopus HKD purse and the ShenZhenTong CNY purse, so you need to add HKD to it in HK, and add CNY to it in Shenzhen. The card is available at all Circle K Convenience Stores and VanGo Convenience Stores. However, you cannot use it to take bullet trains.