I like to cram as many grape nuts as I can into a container of soy yogurt. Not necessarily before a ride though. Wonder if that would be ok??
Speaking in theory:
Soy yogurt will have plenty of sugar, but also protein which will make you feel a bit more full and slow down digestion. Grape Nuts has fiber which is why the XC guys ragged it. But it's also got lots of carbs. So between the two you have plenty of carbs, mostly simple, some fiber and protein which will make you feel a bit more full/satisfied and slow digestion a bit.
In practice I've had no problems with either Grape Nuts nor soy milk separately.