It is possible to have 2 active skills. If you analyse 'Shadow Words' Warlock's skill, it is in fact 2 skills. A heal and a DPS. It's function depends on the target selected, a bit like so
Target Function = Enemy
Target Function = Ally
Trigger All For One
Also, to simplify, they have a shared CD. So if you target an ally, and so triggering All For One, you cannot cast Transmute until the CD is over.
Finally, All For One is exactly what you think in your head.
Let's say it's AP mode 5v5.
I Target an ally with All For One.
For the next 7 seconds, all Allies on the map will not gain any gold, and instead increase the rate the targeted hero gains gold by (0.40 x number of allied heroes) per second.
This also means if an allied hero kills a creep, tower or pwns an enemy hero, the one that kills it will not gain any gold, and instead transfer that gained gold to the targeted hero.
e.g. I kill a creep that gave me a 100 Gold Bounty and the effect triggers, meaning that all my allies are given 15 gold.
e.g. I kill a hero in my lane (No allies are nearby, I'm soloing) and the hero kill message remains normal. But, the extra gold bounty normally obtained by allies that assisted or are nearby are given to all allies, regardless of how far they are in the map.
If a hero does assist in a kill, they will not receive bonus gold from 'One For All!'
Also, Dead Allies will not gain any of the above effects, since they HAVE to be alive for them to have 'One For All!' Aura on them