Separate call/local stacks is not really a property of Harvard architectures, which are distinguished by having different address spaces for code and data. The split is really orthogonal to the architecture type.
IA64 is Von Neumann architecture, but still has separate stacks. In fact, at least with clang with SafeStack enabled, you can have separate stacks independently from architecture.
Of course our a modern CPUs have generic enough instructions that let you do that. Harvard pretty much forces it though, so much so that many Harvard CPUs don't have a way to get code into data registers and vice versa. Since they are separate busses they may be different data widths and therefor incompatible.
IA64 is Von Neumann architecture, but still has separate stacks. In fact, at least with clang with SafeStack enabled, you can have separate stacks independently from architecture.