Hi Muhammad,
Yes, this type of design will always be required at one stage or another. To make one Core fail over to another is the easy part, it can happen already. The challenge is to make one of the Cores go down and come back online safely so that they don't disrupt the network. There are always going to be conditions in a dynamic network where the Cores will go up and down e.g. network/IOS/Software upgrades. So a bit of intelligence between the two Cores is required to make it a supported safe feature e.g. A keep-alive link between the Cores need to be established which will signal and sync state between the two Cores. The design will then look something similar to the attached image. In essence all wan traffic (both routers) is handled by the primary Core. If the primary Core fails the standby Core takes over. The communication between the two Cores will then ensure that the primary Core can safely rejoin.