The grounding electrode conductor (GEC) has to be continuous to any available electrode.
From that point the GEC stops and a connection to any other electrode is a bonding jumper. The connection to the rebar is OK if the acorn clamp is listed for direct burial/rebar/concrete encasement.
But two conductors under one clamp is most likely a violation of 110.3(B)
While a cadweld connection may be desirable, Burndy would like you to know they make a compression connection they belive is superior. Whatever is used must be listed for the application.
