A double with a big difference was straight forward with TA Cyclo-Tourist cranksets,
a friend had a 50-28 on his british bike, on our SF to SD Cali coast trip
White ind has a Clever crankset the Big ring has 5 slots so has the option
of a variety of Bolt circle inside rings as long as there are 5
http://www.whiteind.com/cranks/roadcranks.html
inner: 24-38, outer 38-52