While a chart or calculator is obviously convenient, you can use the skillchain page on bg-wiki.com It tells you how properties react with one another to form skillchains, so you then just have to look up the SC elemental properties of the weaponskills you have access to. Only Empyrean and Relic Weaponskills can make double Light or Darkness, because they have "Light" and "Darkness" as SC properties (distinct from "Transfixion" and "Compression"). The lvl 1 SCs are too complicated to memorize and usually not worth it, but a basic rule of thumb for making lvl3's is that you combine lvl2 properties on the same side of the light/dark alignment to make Light or Darkness, and a WS that has a lvl3 SC property (Relics and Empys) can continue it for an additional level 3(4?), and it stops there. So for example, Atonement(Fusion) > Savage Blade(Fragmentation) = Light > Chant du Cygne(Light) > Double Light. Furthermore, lvl3 SC properties can form a lvl4(?) SC's with themselves, but can't be continued. So Chant du Cygne(Light) > Chant du Cygne(Light) = Lvl4 Light, which cannot be continued. For making Multi-steps that start with lvl 2 SC's, it goes like this:
Gravitation > Fragmentation = Fragmentation.
Distortion > Fusion = Fusion.
Fusion > Gravitation = Gravitation.
Fragmentation > Distortion = Distortion.
Memorizing these patterns, it becomes fairly easy to figure out SC's on the fly with RME and merit WS's by simply knowing or looking up the Weaponskills' pages on bgwiki, which lists their properties. One of the reasons Samurais can make really long multi-steps is because Great Katana can get access to all 4 lvl2 SC properties, so they can make a bunch of Lvl2 SC's before finally doing Light and ending with Double Light via Fudo. Blue Mage cannot solo double light using only swords because they do not get access to any fusion-aligned sword WS's (I believe Atonement is the only one), and therefore cannot make a lvl3 Light Skillchain to follow with CDC for double light.