Please follow documentation requirements:
Each node must have at least two network adapters or network interface cards (NICs): one for the public network interface, and one for the private network interface (the interconnect).
The private network must be configured before installation on a separated network, with its own subnet, that is not resolvable except by other cluster member nodes.
As you have been aware, public ip and private ip must be in different subnet. I don't know why you can not do it. Limited network switch? or something else? I'm thinking at least you can use a crossover network cable to connect your two nodes directly although it's not recommended nowadays (it used to be the option in 10 or 15 years ago). However, your RAC is only test environment, so it should be ok.