See “Whittler” below; this term is a permissible exception to the definition because it is the antecedent technology to modern whittlers. Some whittlers have their springs lying side by side, touching each other. Other whittlers have a tapered divider (See: Whittler, Tapered Divider; also Whittler, Catch Bit) between them so that at the two-blade end the individual springs are held apart. However, a true split-spring whittler has ONE solid spring that is literally split into two parts for about half of its length (from the two-blade end.) True split-backspring whittlers date from the 19th century and are rare.
Lockback Split-Backspring Whittlers actually exist though they are extremely rare, the locking latch is a separate piece but the spring that operates the latch and the two secondary blades is a single piece.
See also: Split-Spring Jack; Whittler, Tapered Divider