You could go either way, A 10 gallon bin can support 2000 worms, and possibly a few more. In a year or less I would think you could have a worm population of about 3000 worms in your bin. That would probably max out a 10 gallon bin. But if you want a second bin, this would be a good opportunity to start it.

If you figure that your worm population will double every 3 months, but not keep growing after the bin is maxed out, your bin will be maxed out in about 3 months. Start the new bin now. Or start one bin now, and split it the first time you harvest in about 6 months.

I had the same thing happen with my redworm order from GardensAlive. I received the 1000 worms (without ever receiving a shipment confirmation), and then received an email confirmation 2 weeks later that the worms had been shipped. I, too, thought I would be getting an additional 1000 worms, but I never did receive the order.
If you are that lucky, though, I would just put them in with the others. It won't be too many, and you'll be able to add more food.