Would it be practical to use two 160 mtr mobile whips
in a horizontal configuration with 450 ohm ladder line
for 160-10 mtrs? Placement of antenna would be 70
feet up from the ground. I realize the efficiency would be
poor but better than nothing. Any alternative suggestions
relating to the above?

Most of the time it is not too efficiant to try and use loaded whips outside
of the band they are made for.