Confusion about how Portal Stones work?

So I thought I understood how portal stones work, but re-reading the flicker flicker scene in the Great Hunt, I got wildly confused.

The issue: In chapter 37 of TGH, Rand and the others are trying to get to Toman Head. They cannot use the Ways, so they decide to use a portal stone.

Portal stones have a top and bottom half. Symbols on top represent different worlds. Symbols on bottom represent other locations within the current world.

When Verin asks Rand what he remembers from his last portal stone experience, he only remembers the (top) symbol for his world. That particular sign isn't on this stone (because they are already in their world.)

Verin then shows Rand the (bottom) symbol for Toman Head.

This is the part of where I get confused

After showing the symbol for Toman head (the place they want to go!) Verin then has Rand pick a random symbol.

Why!?

He already knows the symbol for Toman Head, and he only intends to go to another place in the world he is already in, so he shouldn't need a symbol from the top half of the stone...why would he need to pick a random symbol? Why can't he just use the one Verin showed him?

I feel like I'm missing something really obvious, can anyone shed some light?

The only thing that makes remote sense to me is that Portal Stones require two symbols (one from the top and one from the bottom) at all times to operate. And the random arrow symbol that Rand chooses is on the top half of the stone (that isn't made clear).