Alan,
From what I understand, it usually entails removing part of the original condenser first (an operation requiring the dismantling of the armature, as the points plate bolt actually screws into the case of the condenser), otherwise the duff condenser affects the performance of the Brightspark unit. As the Brightspark is inserted in place of the insulator between the fixed point and the points plate, I guess it is effectively just wired across the points - in which case, an additional condenser wired into the cutout terminal should provide a similar improvement.
At the end of the day, I guess it depends on whether the original condenser has failed due to it short circuiting, or just deteriorated to the point of having very little capacitance.
Regards,
Eddie.