I've run into this problem also and just had to scrap it, honestly. My work around for this is I have created a **** ton of passives that allow for different weapon and skill combinations, with weapon requirements of course. However, you can make a multiple piece set with different unittypes, and make the skill require all of the different unittypes.

Example:
Custom rings: example unittype [Archetype Ring]
custom necklaces: example unittype [Archetype Necklace]
custom sword: example unittype [Archetype 1hsword]

Skill: Archetype Focus [Any affix you'd like to add, including procs and excluding "Quota" affixes]
Skill requires unittype: Archetype Ring, Archetype Necklace, and Archetype 1hsword, and will not trigger without them.

It's a bit of a clunky work around, but it can easily be done very gracefully with a bit of time and polishing. I've done this myself to add stances and passives for all of my new custom weapon types, and a bunch of other stuff.

Secondary, more 'streamline' option:

Make your set attach a custom unittheme named after your set, and make the item require the unittheme. It's less legwork with the same result. This way you could also double up skill usage by simply tagging your custom theme onto virtually any item.

Hope this helps some!

Reminder: these are only necessary if you're trying to attach a skill affix
!!Reminder: you cannot attach "Quota" affixes to set -OR- skill affixes, they won't trigger.

I've run into this problem also and just had to scrap it, honestly. My work around for this is I have created a **** ton of passives that allow for different weapon and skill combinations, with weapon requirements of course. However, you can make a multiple piece set with different unittypes, and make the skill require all of the different unittypes.

Example:
Custom rings: example unittype [Archetype Ring]
custom necklaces: example unittype [Archetype Necklace]
custom sword: example unittype [Archetype 1hsword]

Skill: Archetype Focus [Any affix you'd like to add, including procs and excluding "Quota" affixes]
Skill requires unittype: Archetype Ring, Archetype Necklace, and Archetype 1hsword, and will not trigger without them.

It's a bit of a clunky work around, but it can easily be done very gracefully with a bit of time and polishing. I've done this myself to add stances and passives for all of my new custom weapon types, and a bunch of other stuff.

Secondary, more 'streamline' option:

Make your set attach a custom unittheme named after your set, and make the item require the unittheme. It's less legwork with the same result. This way you could also double up skill usage by simply tagging your custom theme onto virtually any item.

Hope this helps some!

Reminder: these are only necessary if you're trying to attach a skill affix
!!Reminder: you cannot attach "Quota" affixes to set -OR- skill affixes, they won't trigger.

Haxley, thanks for the response.  I was wondering if I could do this with a Unitheme.  I don't really understand them well, so I will have to study what you say, because I didn't really understand it haha   

Hm...

This seems like it should work. If I get any time I may dust off GUTs and take a crack at this one. It shouldn't take too much time to determine if there's an effective workaround.

Epoch,  Thanks. I hope you get it working or find another way.  I really am at a loss on this one.

Do this, it works, I've done it:

Make a new unittheme for each item, name it whatever you want, make it attach to ANY bone with no particle [if you don't know what the bones are, just look through the other ones and pick one to copy, like shoulder or back or something]

Make your passive spell like you want, attach the unitthemes as a skill requirement.

Why use unitthemes: They are invisible! These are what attach special particles TO your character, for example, this is what makes the electricity stick to you and flutter around while you/the enemy is electrified. same with frozen and silenced and etc.

Just name the skill something like '____  Set Bonus' or something, you can still add all of the items to a SET and just don't attach any affixes.

I think you can attach different affixes based on which unitthemes are present, I haven't messed with that yet though.

Anyway, this will lead to items still being shown as set, show how many you have of the set, display set names, however the set itself will get it's bonus from a spell slot passive.

This also addresses a few things:

People could potentially stack and pair sets
People can achieve sets using different pieces of gear [The set could contain say both a sword and a 2hsword that have this unittheme, making them both compatible as pieces of the set]
more interesting combinations

let me know your thoughts

Can you send me an example so I could reverse engineer it?