If only 2 lenses....which combo?

--sigma 17-70 and 70-200
If you like super wide.

--24-105 and 70-200
If you want a light walk around with IS.

17mm on a 1.6 crop is hardly superwide. in ff terms, basically 24mm to 35mm is considered wide, and beyond 24mm is superwide. so in 1.6 crop terms, you really need something wider than 17mm to get into superwide territory.

On Full frame... 24-105L and 70-200L
On 1.6x crop... 17-55IS and 70-200L

But obviously these both cost a LOT. The Sigma 17-70 is nice cheaper alternative.

17-85 IS & 70-300 IS (don't get mixed up with the old 75-300)

I use three different two lens combinations depending on the type of photography I am doing.

For nature and wildlife I carry a 17-55/2.8 and a Sigma 50-500. That gives continuous coverage from 17 mm to 500 mm. The 50-500 is somewhat heavy and bulky for many purposes though.

For dimmer light conditions or fast sports I use a combination of the 17-55/2.8 and 70-200/2.8. Both are excellent quality glass, although the 70-200 is still somewhat bulky and won't fit in a small camera bag.

The trouble with either of those combinations is that I sometimes need a lens wider than 17 mm, so I often make a three lens combination and add the 10-22.

For travel, where I need a compact outfit that will fit in a small bag I take the 10-22 and my 28-135 IS. I considered replacing the 28-135 with the 24-105/4, but have not done so.

There is another two lens combination that I haven't got around to acquiring yet for tavel where I need a compact outfit. That would be the 17-55/2.8 and the 70-300 DO IS. As others have noted, if cost is a factor, the 17-85 IS and 70-300 non-DO IS could be substituted. I am not a fan of the non-DO 70-300 because it does not have full time manual focus override and the front element turns when zooming.

cheaper option:

17-85 IS, 70-300 IS (both excellent lenses)

a little pricier:

17-55 f/2.8 IS, 70-200 f/4 L (the 15mm gap is nothing really, you can walk around to cover that distance).

pricier still:

17-40 f/4 L, 24-105 L
or
17-55 f/2.8 L, 70-200 f/2.8 IS L

I have the 17-55 f/2.8 IS and 70-200 f/4 L combo. Good optical quality and not too heavy for me.

I have the Sigma 17-70 and the 70-200 2.8 EX DC Macro, costing approximately $ 1300 for the pair from B&H. I still have a Sigma 10-20 for the ultrawide stuff. I have a Kenko pro 1.4 TC on order to bump up the reach of the 70-200, figuring that I would have a 96-280 F4 cheaper than buying a separate 70-300.

24-70 and 70-200, unless you need wider than 24mm.

With 1.6x crop, 17-40L and 70200 f4L to cover MOST of the range on a budget

(this is if you wanna stick with canon though... I'd recommend third party lenses, but don't have much experience with them, so I'm just giving you a suggestion from personal experience )

note that those are f/4 though so you may not be able to use it all indoor, but I weighted most of the decision on using the lenses outside

my $0.02....and this is just my opinion (and im a photography noob)

Sigma 10-20
for your occasional landscape, sunset. I have a 18-55mm kit lens but on a x1.6 sensor, but sometimes, it feels a lil too long... Maybe the EF-S 10-22mm if your wallet lets you.

Sigma 17-70 macro or the Sigma 18-50 f2.8
for your kids and everything else. if you need the f2.8 on the long end, then maybe the 18-50mm.

Since we have strayed from the original question, I'll throw out my latest two-lens combinations:For daytime use, the EF-S 17-55 IS and the EF 35-350 f3.5-5.6L. Both lenses pack into a small space, and they give you a great range. I still dislike push-pull zooms, but this is a great combo. The 28-300 L IS raises the price quite a bit, but it may serve just as well.As the dark approaches, replace the 35-350 with the "Magic Drainpipe" (80-200 f2.8L) or the current 70-200 f2.8L (with or without IS).Have Fun,

17-40 and 70-200IS is what I would get if I had only two choices. For the same money you could get several , three or four Sigma or Tamron lenses. I don't own any but many are very good.

17-40 L

and

70-300 IS USM

17mm is about a standard wide in terms of it's field of view. But 17-70 provides a decent walk around(at least the common term used for these focal lengths). You aren't going to get a 2 lens combo like that with a super wide. Something gets sacrificed then.