on a 10 speed crank the chainrings are closer together, but the 9speed chain will work on it no problems.
but, changing my 9 speed triple to a 10 speed triple caused enough problems on my setup that im taking off the big ring as i couldnt get the chain to stay on there on the downhills :( ; the derailluer wasnt in the right place to keep the chain on it. i liked having a big ring so im still not sure how 2×9 will work out.
that was on a tripple though so it may be ok on a double crank.
im tired, so if that makes no sense let me know and ill try to explain it again :-)