water/yeast for barleywine

I'm planning on brewing a barleywine next weekend since I just got married and thought it would be cool to have a beer to share on anniversaries.

To do that, I'm planning on brewing Randy Mosher's Ignoble Doble-Doble:
20 lbs Marris Otter malt
2 lbs amber malt

Questions:
1. I live in area with off-the-charts hard water. Normally, I run all of my water through an activated charcoal (i.e. Brita) filter and dilute it 50% with distilled water, though I've used higher amounts of distilled water (usually for yellow beer). Should I use harder or softer water for this barleywine?

2. I'm planning on brewing up a batch of YooperBrew's Fizzy Yellow Beer on Monday and fermenting it with 1056, and racking my barleywine wort onto the trub from this first batch seven days later. I normally have very good sanitation practices, but I haven't done this before (though I've read that other people have). Is this a good idea or should I think about adding multiple packets of yeast instead?

3. My friend said he made a big beer once by pitching a packet of 1056 AND a packet of Scottish ale yeast 1728. I've never combined yeasts in a single beer. Does this result in greater complexity or does it just add $7 to the cost of the batch?

Thanks for any and all advice!

1) It depends what style barleywine you're trying to make. The traditional examples are from London and Burton-on-Trent. I wouldn't try to emulate exactly a town's profile. I would use distilled to get your carbonate down to an acceptable level for a pale ale. That said, you'll have so much flavor in that beer, the water shouldn't make much difference.

2) I usually wash my yeast when I reuse it. Google "how to wash yeast." I've pitched onto a cake before, but chances are you've got way more yeast in there than you really need, plus all the trub from the last batch. If there is too much yeast, the yeast may not reproduce to get acclimated to the new wort. You want young, healthy yeast, not old tired yeast. Having a couple generations grow in the wort will help it get acclimated to the higher sugars and probably will attenuate better, instead of the old yeast going nuts for a few days, dying/flocculating, and leaving your beer too sweet. The times I've pitched onto the cake, I got massive blowoff in the first few days, it got really hot, and then I ended up with a slightly stuck fermentation.

3) I've only mixed yeasts when I didn't have enough of one for some reason. I could see mixing a hefe yeast and a belgian yeast for a wit, or something along those lines, but 1056 and 1728 are similar (and neutral) enough I doubt it'll make a difference.

I'd stick with your usual 50/50 water mix and do whatever you need to do to adjust the pH (salts, 5.2 stabilizer).

As far as the yeast a 1056 cake is right on. Be sure to aerate well, and add a teaspoon of beer yeast nutrient (I use Wyeast nutrient) to the kettle at the end of the boil, the yeast will need it after having been through a previous fermentation.