Two card rig is not a lot of money. Just mine and make some money for a while then decide in a few months if you want to expand.
I know it's not a lot but when you're an IT student who makes around 100 a month you can't invest a ton
But i really think i will expand my rig later, the only thing that would make me not invest is if i can't get my money back at all
So if you make $4/day now for 2 GPUs, you will make $1/day then. So $0.50/per gpu/ per day.
So it will still be profitable but very low profit compare to now
Rx 470/480s are rather inefficient when mining other algos compared to Nvidia. I suggest having some Nvidia cards or wait for Vega unless your electricity is very cheap.
That's what i was thinking to do after RoI to invest in some nvidia card so i can be "polyvalent" rig