For me, everything's been really surprisingly easy with my two Sennies - but then again, they're both way past their sexual maturing years, they're both friendly and calm and most likely not previously traumatized in any way.
Alaska's very independent and calm. She likes to
see me, but not necessarily be all over me - she'd rather just climb around in her tree or hang out on her swing next to my comuter (where I spend a lot of time.

) and with Miley.... Miley basically never leaves my shoulder.

So they are SOOO different and I'm pretty sure that's a huge reason to as why I pretty much haven't faced any troubles.
Also the fact that
Alaska was my first Sennie, since she's never been, like, glued to me. She didn't mind watching me hanging out with Miley as long as I let her hang out by herself next to us - and since Alaska's always been here (from Miley's point of view), Miley didn't mind Alaska either.
They share the majority of their out of cage-time now but they barely ever actually interact

(Though Alaska reeeaaally loves it when Miley pays attention to her. I think someone might be in looove! ^_^)
And Miley's still soo quiet, she haven't picked up Alaska's screaming! I actually really thought she would, haha.
So yeah, I wouldn't say two Senegals HAVE to be "a recipe for disaster" - though, I am aware that I have been very lucky, and if Miley (the bit more jealous one) would've been my first Senegal, I think she would probably pretty much have hated Alaska, and it would've been so much more work getting them to tolerate eachother.
I obviously don't know Tiki or her brother, but since it seems like the brother is a bit traumatized or so, I think one main thing you have to ask yourself is if Tiki would mind you handling another parrot a lot. I know it's hard to predict problems, so you obviously can't be sure about.. Anything. But I'd say try it out - you might also get lucky.
I'm very much in favor of keeping parrots in same-species pairs/flocks if possible. Here in Sweden, it's pretty much law (well, at least highly recommended) that you keep two or more birds of the same species together.
Don't get me wrong, I don't at all
mind or question people keeping a single parrot, it's all great, but I think it's even
better to be able to keep pairs or flocks. They do, after all, live in flocks in the wild.
Even though Alaska and Miley doesn't really interact with eachother (I mean... Two
female Sennies? Both handfed and used to living alone with people? Even I have to agree that's not ideal!

), I've seen a huge change in Alaska since I brought Miley home.
As mentioned before, Alaska really loves Miley. She respects her wanting to be alone and all, but when Miley pays attention to her, Alaska's in heaven.
She's so much happier in general, too. She'll hop around like a little frog and if Miley's in another room, she'll call for her until she answers. When she's answered, she'll go on playing like normal - kind of like she wants to be sure Miley's okay before she can continue doing her stuff.
Before Alaska moved here, she'd been clipped for a while and didn't really want to fly unless she had to.
And then Miley came along. Miley loves to fly. She'll fly anywhere - everywhere, all the time!
"Oh gosh, that looks so much fun!", Alaska thought and tried it out.
She's now flying around happily, more and more. I love it! She loves it, too.
I'm certain this is at least partly thanks to Miley.
Oh, and not to mention the fact that Miley was given to me because her previous owners realized she might be better off in a household with another Sennie, since they noticed a huge change in their Caique when they got him a Caique-friend.
I'm sorry if this wasn't helpful, I just wanted to share my thoughts on keeping two or more parrots together. (And as I said before, I know it can be really hard to introduce parrots to eachother and everything, so I REALLY don't judge anyone who keeps single parrots.)
I'm not saying everything will magially become great, I'm just saying things aren't automatically doomed to turn out awful.
I thought a bit enouragement might be nice, because while you SHOULD ALWAYS expect the worst, maybe the worst doesn't HAVE to happen, and that could be nice to know.
If I had turned down Miley (And I was about to! Two female Senegals together didn't really sound that wonderful and I was so worried. And on to of that, Miley was said to be aggressive!) I would've never known how good things actually did work out.