Do I really have to do Valtan AND Vykas in order to craft the Nightmare set? No other way?
you can do brel as well but you wont be able to gear transfer legendary gear into brel which means you would have to hone it from 0 so for your case probably yes you need both
I think you’ll save some weeks if you hone legendary to +20 and do valtan or vykas and brel + clown. I may be wrong tho, haven’t done the math.
That’s only if you have the mats. Otherwise do them as you can, had no idea you can’t transfer legendary to upper relic.
technically the best way if you are 1490 immediately is to do valtan, vykas, brel, hone brel piece to hit 1500, then do g34 brel and that way if you take bid boxes and correctly gear transfer and dismantle you can generally hit 6 relic pieces in the 2nd week, after those 2 weeks you do clown and brel and alternate between vykas or valtan every week to unlock the set conversion
OP didnt want know whats the best way
he wanted to know if he has to do both raids for a full nightmare set
answer is no
you can instead of doing valtan and vykas ,do Brelshaza as an alternative
finding a Group who takes you without a full 6set will be pretty hard . b ut you can buy busses, im sure they are cheap next week 
but yes you could skip valtan or vykas for Brel if you hone your legendary set to 1490
i said that in the first post and it will make him end up honing brel pieces from 0 instead of 11+