Multiple OBD connections?

Hello,
Is there anything wrong with having OBD sessions for multiple ECUs (Electronic Control Units) simultaneously?
Can it, for example, connect to both the engine control unit (ECU) and the speedometer simultaneously?

In principle, no, it depends on a) the protocol used, b) the physical layer, and c) the topology within the vehicle. All three must allow for multiple 'connections'; otherwise, it won't work.

I meant to ask if it is explicitly "prohibited" by any standard, guideline, or similar document in the automotive industry.

Technically, it works. I just tested it on an A3 by accessing the CAN bus of the infotainment system. I established an OBD connection to both the engine control unit (ECU) and the instrument cluster control unit (ICU) and read data from both measurement blocks. Okay.

The standards, reference procedures, etc., explicitly address this and provide the relevant guidance that should be considered when designing a system for multiple connections.
Similarly, the logs explicitly mention this use case and provide relevant information.

'Forbidden' is therefore nothing.

Ultimately, whether it functions reliably and whether the system has been sufficiently tested for this specific use case remains to be seen, as experience has shown. Then, it may not only be the communication that stops...