The increasing adoption of IoT devices highlights the urgent need for robust security solutions, particularly on low-end MCUs. However, these platforms typically lack the advanced security mechanisms found in higher-end processors, making them highly vulnerable to attacks. This paper presents an interoperable solution that enables Trusted Execution Environments (TEEs) on low-end MCUs, leveraging platform-specific hardware security primitives available in both ARMv7-M and RISC-V architectures. By complementing these primitives with software-based isolation techniques, the proposed solution achieves strong domain isolation even in resource-constrained devices. Additionally, the adoption of the GlobalPlatform API provides a common abstraction layer that decouples application development from platform-specific details, allowing trusted applications to run seamlessly across different MCU platforms. In the evaluation, we assessed the solutions through performance microbenchmarks and real-world applications, demonstrating their feasibility and the trade-offs between security and performance across different workloads.