PERPdemo.py

import bpy
import bmesh
import mathutils
import math
from mathutils import Vector

obj = bpy.context.object
me = obj.data
bm = bmesh.from_edit_mesh(me)

verts = [v for v in bm.verts if (v.select and not v.hide)]

#if not len(verts) == 2:
#    msg = "select two vertices"
#    self.report({"WARNING"}, msg)
#    return

v1, v2 = [v.co for v in verts]
print('vectors found:\n', v1, '\n', v2)

mid_vec = v1.lerp(v2, 0.5)
plane_no = v2-mid_vec
plane_co = mid_vec
dist = 0.0001

# hidden geometry will not be affected.
visible_geom = [g for g in bm.faces[:]+bm.verts[:]+bm.edges[:] if not g.hide]

bmesh.ops.bisect_plane(
    bm, 
    geom=visible_geom, 
    dist=dist,
    plane_co=plane_co, plane_no=plane_no, 
    use_snap_center=False,
    clear_outer=False,
    clear_inner=False)

bmesh.update_edit_mesh(me, True)