Generally speaking, you want your slayer tasks to last as long as possible. If you don't want to do the task, you cancel, and if you get bored half way through doing the grotesque guardian bosses and don't want to cancel, you can just swap over to doing regular gargoyles. What the double trouble unlock does is cut the number of kills in half, with no perceivable benefit.
One simple solution would be that you get slayer exp for both dusk and dawn separately, rather than together. Overall you'd get the same amount of exp, but at twice the rate. It's already a pretty slow way to get slayer exp, so it's not going to change the meta, but at least it would make double trouble worth unlocking for 500 points!
Any other suggestions around this, or am I missing a major advantage I'd overlooked?