To run multiple instances of TEE (or multiple trusted OS), we need proper resource partitioning in secure world which could be achieved via some sort of hypervisor in secure world like we have in normal world (EL2).
Secure EL2 (secure virtualization extensions) is added via recent ARMv8.4 architecture  which is not supported by hikey620. And I am not sure if there is any publicly available SoC supporting this secure world extension.
Also, current open source software software stack (TF-A and OP-TEE) doesn’t support multiple instances of TEE. But it seems to be WIP, see .