If you are running an X session under Ubuntu on machine A it will appear on screen:0 (Ctrl+Alt+F7) on A.
You want to run a remote X Session from machine B (on your Lan) without interrupting your current session on A.
Go to the first console screen (Ctrl+Alt+F1) login at the console and then ask Machine B to send an X session to screen: on A - like this
u@A:~$ X :1 -query IP-of-B
X asks for an X session,
:1 says to put it on screen 1,
-query asks for the session from in this case IP address of B
You will find the new session on screen :1 (Ctrl+Alt+F8).
This is not secure (no ssh) but is quick and easy for a home network when sound is not required over the connection.
You can use nomachine's nxclient/server which is fine but runs in your current session on A screen :0 in its own window. It provides support for local sound and local printing - last time I looked Firefox would not start up if sound was enabled.