Premixing fertilizer
 

Is it a bad idea to mix up fertilizer and leave it for the next working in a couple of weeks?

I think it's not too bad as long as you don't get precipitates forming. Best way to do that is to only make up the weak solution you need, and not a concentrate.
Also store the bottle in the dark to stop algae forming.

I have been filling gallon jugs of water with weak fert for a few years with no ill results. I keep them on a shelf under my light garden, which prevents algae from growing. I usually go thru 4 gallon jugs pretty quickly, and refill them all to be ready for the next watering.