I use Stm32 and recently upgrading my design. Current product uses built in USB full speed and I do everything via this port. (Fw upgrade, service, other usual use cases).
In the new design marketing asked for Hs USB and I will add a ulpi phy to make stm32 Hs capable but there is a catch. The Hs operation doesn't allow dfu etc.
So, can I connect the dp/Dm from ha USB to FS port and use them together. The only time the FS port will be used is during dfu where the ulpi won't be working. As soon as the CPU boots I will make FS port high impedance so that it doesn't interfere with the Hs operation.