Current Game Bug issues - 1/6/2013
It can’t stack.
http://wiki.guildwars2.com/wiki/Sigil
On-kill sigils:
On-kill stacking attribute bonuses remain even after you swap or unequip weapons; the stacks only end on being downed or defeated, by traveling to another zone or entering an instance.
If you have an “on-kill sigil” on both main-hand and off-hand weapons, only the sigil on the main-hand weapon will trigger.
Two different on-kill stacking sigils will not provide bonuses to two different attributes. Whichever sigil triggered the first time will continue to add to its stack. (i.e. you can only have one “on kill” stacking effect active at a time). However, if you have the same on-kill stacking sigil on both main- and off-hand weapons, stacks will accumulate twice as quickly. The 25 stack limit still applies.
The same limitation applies to mixing Stacking on-kill sigils and Delayed on-kill sigils. Only one effect will be present.
In sPvP, stacking and delayed sigils gain five charges per kill, i.e. they require only five player kills to reach the maximum of 25 charges.