If you're at the stage where you can instakill your way to the high 100s or 200s, then double trouble will actually give you quite a significant amount of more idols/chests an hour than freeplay of either campaign.

Why? Well quite simply, the spawn rates are doubled, however the objective to move to the next stage remains the same. "Collect 10 objects" or "kill 25 enemies" etc. The drop rate of the objects is also unchanged. This means if you're instakilling, you'll progress to the next map pretty much twice as fast (Though stuff like moving to the next screen, or the auto scroll picking up the items is unchanged, so in practice it's not quite double)

The downside is the double bosses, it takes a bit longer, but you save a lot more time doing the maps faster than fighting the extra boss.

So yeah. Just a word of advice. Don't finish the challenge, as you can't replay it, and then you lose the best way to grind idols and chests in the game right now.

Edit: Not only is it faster, it's a hell of a lot more convenient to quickly reach the stage where you have your full party and upgrades such as panda's gold bonus, and can just leisurely focus on buying X25 of your DPS guy.

Hoping the devs do something about it cos it doesn't feel quite right.

Just let us replay the campaign without Rubies reward and it will fair for everyone.