Is it possible to submit an ETN in which some of the fellows receive a joint degree? And is that an advantage for the evaluation?

It is possible that some fellows in ETN will be enrolled in joint/double or multiple degrees, but it is not a formal requirement as in EJD.  As such, joint degrees are not considered separately in ETN: the proposal overall will be evaluated against the evaluation criteria of ETN.  It is important to keep in mind when submitting an ETN that  minimum eligibility rules and other aspects of that particular mode must be respected.  In addition, ranking takes place within the ETN mode.