You may set the first one as Access Point mode which can be connected to your router through wired network.
Then, set the second one as repeater to extender wireless network from first TL-WA5210G.

Simon wrote

You may set the first one as Access Point mode which can be connected to your router through wired network.
Then, set the second one as repeater to extender wireless network from first TL-WA5210G.

wouldn't that change the SSID? i need the same SSID for all of the network and the router has dual band antennas.

use same SSID and same security and passwords, but that would not make a dif. this toys wont let you do roaming,
example, with a Iphone, 3 floor home, you are on floor 3, getting signal from a 5110 on same floor, you move down to floor 1 and there is another 5110, your iphone wont change to that 5110, as
long as there is a bit of signal from 5110 from the third floor it will connect to 5110 on first floor untill you loose signal bars from 5110 from third floor, or change manually, as long as you use 5110s there is no
roaming.
And with Windows OS its worst.

so I understand that the best way would be to disable WiFi on the router (or use separate SSID) and configure the 5110G as an AP and the second as a repeater, right?

I don't see why disable wi-fi
Simon answered on post 2, can't help you with Repeater mode, have never used but the how to here http://www.tp-link.com/en/article/?faqid=151 ignore model tutos are generic