CentOS - Jan 2017 - Can/should I mix entropy sources?

I've got 2 sources of entropy in my CentOS7 system, a hardware RNG and 
the Intel rdrand instruction.  Is it advisable to just use a single 
source or can/should I mix the 2 sources?

If I mix the 2 sources how is it done?  Presumably I could use the rngd 
service to handle the Intel source, but would it be a problem if 2 
entities were writing to the entropy pool?

Thanks.