Question

are 2 parallel lines always coplaner?

Answer 1

If there are only 2 lines then yes, they are always coplanar.

IF there are more than 2 lines it is sometimes coplanar

Answer 2

the cross product of parallel vector produces a normal vector of (0,0,0)

so a plane the contains all the points of both lines can then be formed as:

0(x-xo) + 0(y-yo) + 0(z-zo) = 0, which is always true ...