Good eve,
You should be able to connect a BC2300 to a BL 3500, pluging a Beolink Converter to the AUX and POWERLINK of the 2300 on one end, and a masterlink cable to the BL3500 on the other.
In that configuration, I'm not sure that you can connect other speakers to the BC2300, as the Powerlink connexions are already used for the BL3500....
You will find the Beolink converter's User guide in the manual's section of Beoworld.
Hope that did help,
Cheers, Laurent.
Avant 28 DVD, BC2, 2300, BL2500, 8000, to name a few...