Yes, first I made the design with a pair of bearing but I ended up with this because the workshop where I make the parts does not have precision machinery and the bearing alignment would have been difficult. Even with this design I fear about pulleys alignment. I don't get why the spaced bearings will increase the critical speed?

The bearing is a Renault rear wheel with balls (most of the makes are with roller). There are two inner rings so if there will be play I can take one out and grind the inner face and then preload like a pair of ac bearings. Am I correct?

Here is the entire assembly. Tomorrow morning I'll go to machine them