I'm thinking of having a dual-ring setup on my hardtail so that I can do more XC. Something like 24 and 42 tooth. But is there a maximum tooth difference you can have between the two rings?

Not sure what the maximum spread between chainrings should be/can be, but anything too big and you're relying heavily on the front mech to do the work, rather than the ramps/pins. As a result, shifting may not be particularly smooth or as sure as, say, 32/44 from middle to outer. Just something to bear in mind.

I agree with Jon. If you're having 2 rings the bigger the gap the harder it is to pick up and the more likely it will be to de-rail on a down shift and come off on the inside.

Choice of rings will help. Shimano have the best ramps & pins IMO but you'll have trouble getting useful sizes. I think in LX spec you can get 26/36/46 and others are usually 22/32/42 or 44. I think the best compromise of these would be 26/36 or 26/42. Depends how much you need the big ring. If it were me I'd probably go 26/36 for the best shifting and get a 11/34 cassette. Should be a wide enough range of gears.

A compact chainset on a road bike is often 50/34, giving a 16 tooth gap. This feels like a huge difference, ignoring the shear logistics of getting the chain to change between them. I'd suggest you may find that a bit of a large jump. Think dual ring set ups are usually a 28/42 or similar, may be making that up though!

Standard MTB chainsets are 22/32/44, cheap ones are often 22/32/42 and non compact are usually 26/36/48.

hmmm I think 24/40 would be ideal, you reckon I can get away with this?

Oh and I will be runnin an 11-27 roadie cassette.

Middleburn do a Duo chainset.

I am aware of this - a new one is out of my budget though. These can come with 29 and 44t rings.

I run 22-32 & only struggle to keep up on flat road sections

I thought about a 2x9 setup myself not so long ago. What I wanted was something like a 34/46 or similar, which would give me more than enough options through high & low gearing for my riding style. I'm still undecided though, as I could still use a 3x9 setup and throw on a cheap but light alu granny, just to eliminate the risk of the chain slipping off completely.

The roadie cassette at the back won't make too much difference to the shifting at the front IMO, just smoother changes across the rear cogs.

What I found when riding a 2x9 setup not that long ago was that changing from middle to large chainring at the front is certainly possible, but you may need to take a bit more care when doing so. I had real problems changing when riding over roots, rocks, etc. In other words, the best pickup was done when 'cruising', with minimal tension through the crankset.

Ok good stuff thanks. It's a shame I can't test it put really... if I buy all this stuff and it doesn't work I'm a bit shafted :-/