Hmmm. Well, I suppose that makes sense, but you would never play two
sources at once. So I suppose we are both right in that there is no
potential harm as long as you don't play both sources at once (your first
answer gave the impresson that merely Y-ing two sources together may be
dangerous), but if you don't trust yourself and you value your gear, you
may
not want to take this chance.

Yeah, I don't think it's a big chance anyway. Most output devices are
capable of withstanding shorting the terminals, I think. And that's
basically what you'd be doing if both were going at the same time. I'd
actually be more concerned about noise/hum that could be introduced in this
manner.

I know it's better to be safe than sorry, but in the real world, wouldn't
you need two high-voltage sources playing EXACTLY the right sine waves at
exactly the right time (at full blast!) to cause damage? I say this
because
I have Y-ed tons of stuff in my life with no problems nor have I ever
heard
of anybody frying anything because of Y-ing.

No, it doesn't really matter what they're playing. I just used the sine
wave example to illustrate my point. If you've got two output devices with,
say, 100 ohm output impedances (typical) and an amplifier input of about 10k
ohms (also typical), then what would normally be passing less than 1mA of
current would then be passing more like 10mA. Like I said, it's probably
not enough to cause damage in most cases, though some output devices may be
a little iffy.