You can't wait to hear back from the other exchange and see that the leg is available, "pure" arb teams in HFT typically quote on both products of a pair (e.g.

You can't wait to hear back from the other exchange and see that the leg is available, "pure" arb teams in HFT typically quote on both products of a pair (e.g. cash vs futures FX), at favorable prices (e.g. I'll quote a bid at X on A if I can aggress at >= X/the implied price of X on B) and turn around and hit the other product as soon as they get filled. Sometimes they'll miss, so they're really running stat arb strategies, but that's the closest you'll get to pure arb in this space.