Hello,

Combining multiple line charts into one single chart will depend on which type of chip creating your Trellis mode:

In case it's a blue chip - moving it to the Color drop-zone will result in one line chart with different line colors. (image 1 below)

In case you have multiple measures (which means Values chip appears in the Trellis drop-zone) - you'll have to drag each measure as an additional primary into the Values drop-zone. (image 2 below). Please see our help site for more information. 

Hope that helps.

Asaf