You can splice both ends of all the double braids, you just have to let the core and jacket balance (milk out) before you do the second one. You could do it before, but the rope wouldn't necessarily be at full strength because the core and jacket will stretch at slightly different rates.
My first PI rope milked about 2'. My next one didn't milk at all, the last one milked about a foot. Both of my Velocity ropes milked differently too, the last one only 4". I haven't owned a full tachyon rope but used 120' to make a couple of rope swings and some lanyards. The green stretched more than the orange... go figure.
Tachyon is tight, compared to Velocity or PI, the splice is a biotch because the jacket is so tightly packed. I'd rather splice used Velocity than new Tachyon.
I got a free NE Ropes hat too... in Hartford, it's kinda trashed now. The guy at TCI in Baltimore said he would send me a new one... I need to give him a reminder. HRC is my favorite hitch cord... although Ocean Poly is giving it serious competition. NE Super Braid (from Sherrill Tree) is my favorite bull rope for natural crotches, super tough stuff.
To back up for a sec, once you decide on a 7/16" (or thereabouts), 24 strand, double braid rope... your choices are all good, it gets down to what you like. Obviously it would be ideal to climb on all of'em but lacking that, you'll be able to use any of them once you get a good hitch tuned in. You can always try something different next time.