How can a switch / hub allow two devices to connect to Xbox Live
They still have to share the internet (broadband) connection. Unless you are rich enough to have two internet services to your property, the cable modem (or ADSL) needs a router to share the connection. This can be wired only or a wireless router (which normally includes wired ports). OK if your router doesn't have enough LAN ports to it you'd add a switch (or hub) to extend it, but if you are already having poor connection issues extending your network can only compound your go slow by adding even more traffic to it.