What USB-2 cabling hardware do I need to connect two Windows 10 computers together?
This is a commendable desire to emulate your device in development to proceed ahead with software development before the hardware is ready. It is a challenging problem. But you can't simulate a USB device on a regular Windows host. USB host has special functions of "hosting" the bus, which are incompatible with USB slave device functions. No "cabling hardware" can help here. The host functions are hardware-based, usually adhere to strict hardware specifications (Intel EHCI or xHCI), and cannot be turned around.

Some tablets/laptops may have an USB OTG (or dual-role-data function), and act as USB devices if connected to another USB host with correct OTG cable. To make this happen, the system must have an alternative USB device controller, another piece of hardware. However, these OTG Device controllers usually have hardware-predefined structure of endpoints (usually implementing MTP or Mass-Storage Class), and it is very unlikely that the built-in configuration will match your proprietary "embedded EFM32 ARM" enpoint structure, or can be re-programmed. 

If your team is designing some proprietary "embedded EFM32 ARM" USB interface, which doesn't follow any predefined USB class, you are out of luck regarding software development in advance. While there are environments that allow virtualization of the system, you will still need to develop a transaction-based model of your "embedded EFM32 ARM" device, which will likely take time, be inaccurate, and can't be trusted. The more natural way would be to get a EFM32 development platform from Silicon Labs, and modify their existing codes to mimic your "embedded EFM32 ARM" device where possible.