1) open both of your molecules in separate windows, copy benzene by edit-->copy
2) in the window of biphenyl ring, past your molecule by Edit-->past--> append molecule
3) to set the position correctly you should use keyboard and mouse
holding (shift+alt) will move only that molecule which you click.
holding (Alt) and moving mouse will rotate only that molecule which you click.
4) move and rotate your molecule using keyboard shortcuts and place your molecule at your desired location make/remove bond by builder--> modify bond and then clean the structure (Edit --->clean)
5) now you can optimize. after which you get the desired structure