It may work, but there are a few things you may want to consider first.
A silver aro may or may not be agressive towards the afro. Also I'm not sure of the growth rate, but I'm sure the silver will outgrow the afro. I'm not positve on this though.
Also you need to be prepared to feed lots of blackworms and other small foods to the afro until it is about 10'' considering it's filter feeding habbits. Plust you will need a different food for the silver.
So IMO it may work, but I would just choose one or the other.