@quangtran1
Have you figured a way to prioritize which router it should grab a signal from? Mine seems to default to the one further away and the wifes picks the closer one.
From past experiments, I've come to accept that a device will connect to the access point with the strongest, unsecured signal first. However, if there was a previous established connection to a passworded access point, that now takes precedence. In other words, a device will connect to an AP with which you have matched the security key first, even if it has a poorer signal than an unsecured AP. There are many Market apps that are supposed to set wi-fi priority for you like Wifi Ruler, SortWifi, etc, but I don't believe they're that reliable.