Set up the second AirPort Express as a router. That means changing the setting for Router Mode in AirPort Utility from "Off" (Bridge Mode) to DHCP and NAT.
That will produce what is known as a Double NAT notice, but Apple gives you the option to "ignore" the error/notice so the second AirPort will display a green light instead of an annoying blinking amber.
You may need to power off both Expresses at this point, then start up the first Express that joins the network, then start the second Express to establish a new connection.
You will now have up the 50 wireless connections available.....and.....your neighbor will no longer be able to "see" the devices on your network (except for the Express that is joining the network). Conversely, you will no longer be able to "see" the devices on your neighbor's network, so both of you will have better privacy and security.