MTB 1x vs 2x cross chaining I've got a good deal and bought a new bike with full Shimano XT groupset and 2x Drivetrain which I am not fully satisfied with, it has 34/24 and 11-42 setup.

I would prefer 1x more but not going to invest in transformation at the moment.

My question is - as it's not a good general practice to combine lowest-lowest and highest-highest rings (I'm coming from Roadies world), will it be bad to use 34T in front in combination with 42T on rear?

I'd rather not to use the front derailleur at all (would be happier with dropper instead).
The same rules apply to mountain bikes as they do with road bikes when it comes to crosslining the chain. You can agrue it's even more important on mountainbiking as the chain bounces more on rougher terrain. 

When you change to a 1x system, the front chainring should have a chainline in between the current 24/34. So the crossline is not as extreme as with 2x systems, when you are at the lowest and highest cogs on the back.

So the answer would be, it's not good practice. But it might be fine. You might experience more chain wear and possibly more chain drops. 

To turn into a 1x, all you would need is remove the front shifter, the front mech, the 24/34 chainrings and replace with a 1x narrowwide chainring. Which chainring you need depends on your crankset, the number of gears on the rear, the rear hub spacing. 

Also see this disscussion on 1x system crosslining