I believe the bilateral factor pertains to conditions such as bilateral sciatica, which would be one condition affecting 2 limbs (legs).
§ 4.26 Bilateral factor.
When a partial disability results from disease or injury of both arms, or of both legs, or of paired skeletal muscles, the ratings for the disabilities of the right and left sides will be combined as usual, and 10 percent of this value will be added (i.e., not combined) before proceeding with further combinations, or converting to degree of disability. The bilateral factor will be applied to such bilateral disabilities before other combinations are carried out and the rating for such disabilities including the bilateral factor in this section will be treated as 1 disability for the purpose of arranging in order of severity and for all further combinations. For example, with disabilities evaluated at 60 percent, 20 percent, 10 percent and 10 percent (the two 10's representing bilateral disabilities), the order of severity would be 60, 21 and 20. The 60 and 21 combine to 68 percent and the 68 and 20 to 74 percent, converted to 70 percent as the final degree of disability.
(a) The use of the terms “arms” and “legs” is not intended to distinguish between the arm, forearm and hand, or the thigh, leg, and foot, but relates to the upper extremities and lower extremities as a whole. Thus with a compensable disability of the right thigh, for example, amputation, and one of the left foot, for example, pes planus, the bilateral factor applies, and similarly whenever there are compensable disabilities affecting use of paired extremities regardless of location or specified type of impairment.
(b) The correct procedure when applying the bilateral factor to disabilities affecting both upper extremities and both lower extremities is to combine the ratings of the disabilities affecting the 4 extremities in the order of their individual severity and apply the bilateral factor by adding, not combining, 10 percent of the combined value thus attained.
(c) The bilateral factor is not applicable unless there is partial disability of compensable degree in each of 2 paired extremities, or paired skeletal muscles.