I can read this in doc/README.developer:

The stack, by default, supports one datalink layer at a time, and uses a
macro defined in config.h or your makefile/project to choose the macro
functions defined in datalink.h.

Which make me think that the stack should also support multiple datalink at once, despite it not being the default.

But when I read source code, I see a static Datalink_Transport in datalink.c, used by datalink_send_pdu. My first understanding is therefore that all messages sent by the stack will flow through a single data link, defined by latest call to datalink_set.

Am I missing something?

(note: my idea in mind is a BACnet server listening on both UDP and MSTP, and I guess this requires a bridge from one to the other or from both to a shared third)