The MDD is an old mini-dash we used to sell. It was designed for use in steering wheels. The CAN stream for it is quite complex and not really any use to you.
A CAN bus is designed to have large numbers of devices connected at the same time, there is no reason the CANoe can't be connected to the bus when you are using ECU Manager.
I would check that you have the wiring correct, your CAN terminating resistors are correctly placed (1 or 2 and no more), you have the correct CAN rate set in all of the devices and that you aren't transmitting conflicting CAN messages with the CANoe.
There is a list of CAN addresses that ECU Manager uses for communications on our website, you can download it here