We just used a brown punky (You can get it on Amphigory or Cosworx) and we didn't need to do any extra wefting. And yeah, we just did a hairline like the Zack wig, just using excess hair that we cut from the back.
I suppose it would depend on the wig base you use. Punky wigs have frizzed roots that make spiking a lot easier since you're less likely to see any gaps, while most other wigs don't have that (which is another reason I combined wigs for Cloud). Having a wig with double the thickness is wonderful for spikey styles even if you don't have the frizzed roots, but I generally will just stick to a base like a Punky if I can find the right color since it's cheaper and takes less time. Also, the amount of hairspray you use can effect how thick the wig looks (if you use too much, it clumps the fibers together more and make it look less voluminousness than it could look with smaller amounts of spray). I think having a 2-wig thick wig would be fine for Terra, though it isn't completely necessary in my opinion :3
Thanks so much for the detailed reply :> The wig I'm looking at is the Jareth from Arda, which has frizzed roots. If I'm using 2 wigs should I avoid a punky wig or is that still my best bet? Sorry I have so many questions, you just do such excellent work I want to learn all I can while I have the chance xD
Oh yeah, Jareth is very thick and would work great! (it's basically like the Vegas we used for our Zack wig, but longer). Well, frizzed-root wigs are generally a lot harder to combine except for the back of the wig.