Is it possible to setup a secondMI424WR Rev I (the red ring one - I already have a second, we moved in with the inlaws) as an access point (for both wired and wifi) by putting a splitter on the line coming from my bedroom wall with one out going to the Set top box and the other out going to the router?
i.e The signal in the bedroom stinks, so I'd like to have a wired connection to the pc and wifi for xbox, phones and tablets
If so, what do I need to do in the second router to make it work?
Basically what you have to do is turn off the DHCP functionality on the second Actiontec to convert it from a full blown router into an AP.
That answer is incomplete. In addition to turning off the DHCP server, it is VERY important to disable the WAN port, otherwise the two routers will fight for control of the WAN connection. The link that gs0b gave has the correct instructions.