Because when you have 2 pieces you need there would generally be an income of both. If you have 2 pieces that have the same income as the one we are replacing it allows us to gear "twice as fast".
Part of the problem with stun guns is that they are needed at consecutive levels and then again as part of a piece in the following level. This means you have an income that is only used on a single toon for an extended period of time.
If it was 2 then once you broke threw the first level the next level could be something you have been gaining from raids or other sources while farming stun guns.
The bottleneck is a pacing element that is placed there and I dont expect them to change it, but in theory if it were 2 peices it would help.