Apple extreme bridged to apple extreme (Windows network)

Using a wired connection:Using the Airport Utility, set up the extreme wireless with the same network name, security and password as the 2Wire but use a different channel. On the Internet Tab, change Share a Public IP address to OFF Bridge Mode. Click on Update Button.
Using a wireless connection: You will need 2 Apple devices, at least 1 Extreme and the second can be either an Extreme or Express. Replace the 2Wire with the extreme and using Airport Utility, set to "Allow this Network to be Extended". Set the second to "Extend a wireless network".

I assume you have a Mac - which will have Airport Utility in Applications/Utilities.
The idea is that the Airport nearest the internet wall connection is the 'base' and the other one is the satellite or 'repeater' - essentially boosting and sending the signal to those devices on that floor.
Use Airport Utility to set the satellite base station (the one not connected to the gateway) (go into manual set up and select the Wireless button). In the first drop down, select 'Participate in a WDS network'. Then update to restart the Airport.
If you are unfamiliar with using Airport Utility, then I'll post step by step instructions. Good luck!

Use the Airport configuration utility. Setup your main base station (the one connected to the 2Wire modem). Then go configure the downstairs one using the same Apple Airport Utility. Bridging should be an option, but is not available on all Extremes. Bridging isn't listed as bridging per se, but as "I want to extend my network" IIRC.

Hi,
OK, do you have the latest version of Apple's Airport Utility: Version 5.4.1 (541.9) ?
If you don't, get it... it's got an excellent interface that will walk you through bridging two wireless routers together.
Once you get your hands on it, you will need to choose which airport will be the primary and which will be the secondary. Ideally, pick the one connected to the highest speed and most reliable internet access.
Connect to the secondary airport and configure it as bridge to the primary, the Airport Utility will walk you through it step by step.