Thank you very much to all. So, in the case of a standard Shimano compact crankset, by just removing the big chainring would let the small chainring too close to the bottom bracket, wouldn't? Would I need then some washer to center it, of those that you have to mount for a triple crankset? I think this perhaps would do that the chain would jump to the inside. There are square taper cranksets of different spindle size, would a longer type avoid this jump?

I live in a very hilly area, but I have in mind this is my second bike, I have another full road bike. A 46/34 setup would be a bit easier than a 34/25, enough for most of the hills, the main disadvantage is the 46/11 would force me to coast some more in descents, not much, in flat I can only move a 50/14-15, equivalent to 46/13. I really don't mind very much the two teeth jump between gears, I already have done lot of miles with it, mainly using only one chainring. I mounted the bike with the flat handlebar and the 9 speed rear casette and long derailleur some time ago.

Taxi Rob, my setup wouldn't be really a work of art, but if I success I will post some picture.