Offset 2 side at once.
 
  File name : FF2.LSP

  It is bother to do offset twice because plate thisness is thin. 
It is difficult select line after offsetting once because distance of center line and first offseted line is too near 
And We must change line type from center line type to solid line after offset.
This program does offset twice and changes line type. You just select center line and enter thickness of plate.

 
Example
Offset distance<15>: 10[Enter offset distance. (Distance from center to entity line)
                                                   in this case, plate thickness is 20mm)]
Color number<7>:              [Enter color of line that be offsetted(default is white)]
Continuous/Hidden/CEnter/Dashed<Continuous>:
                                [Enter line type (default is continuous)]
Select object: {p1} {p2}        [Select center lines. you cad use cross box]
Select object:                [Type enter to finish selecting]