This may be too obvious but the addition of 4 straights allows one circuit to fit exactly inside another. The absence of a crossover may make this appear uninteresting but it is easier to control. Also one circuit may be from the 12V system and the other from the 9V system, so the two systems can be run side by side.
| 1 | x 4515 |
| 4 | x 4520 |