Yes, they could stream both.  However, the more you stream, the higher your C1 cost gets for that period.  Don't forget, they still need to get the metal out of the ground and can't use the credits twice.

Per example: if the C1 cash cost is $1 with no stream it could become $1.50 when streaming gold/silver.  So you are a lot more dependant on copper prices, but I believe copper is a good bet for the next decades.

Moneyk