Just give them the same SSID and wireless security information. Your wireless client will treat them as if they're one and the same. Whichever has the stronger signal at the moment will usually make the connection.
In fact, this is exactly what happens (when you don't want it too) when neighbors install the same make of wireless router, then naively use the default SSID (e.g., linksys) and leave wireless security disabled. Before you know it, one or the other is mistakenly using the other's router!