It's old question but i can't find any specific answer for this. Me and my friend debate about this. We have 2 opinion:

1. With "watch this", if you commit additional 3 resource, you gain 6. And with Double or nothing, you gain twice. So the process is :" you spend 3 resource (go in supply), you success the test, you take back 12 resource.

2. With "watch this" you spend additonal 3 resource and you gain 6, it mean the test gi e you 3 additional resouce and you can only double those 3 additional resource. Process is:" you spend 3 resouce (go in to supply), you success, and you only take back 9 resource".

Can you guy find some official information that could explain what is right ?

Option "1." is the only correct possibility. Option 2 is overthinking things.

The process is this: you commit Double or Nothing and "Watch This!", and spend 3 resources as part of the cost to commit the latter. Later on, you pass by at least 1. You gain 6 resources from "Watch This!". You then resolve the effects of succeeding a second time, gaining another 6 resources. Nothing about Double or Nothing requires you to spend costs a second time - just all the results of success (everything that resolves in step 7 of the skill test) are repeated.