I think this depends on the actual formulation of the drug, right?

For example two of the titrations of Concerta (XR methylphenidate) which I have experienced first-hand are a hard candy-like tablet, and the pharmacokinetic result is that you receive 25% of the dose within a couple of hours, and then over the course of the rest of the day the remaining 75% hits your bloodstream.

But I now take an XR lisdexamfetamine which looks like it would be possible to split into smaller doses, but it's a prodrug so it's a totally different experience.

I know it's a luxury some folks can't afford, but there really are a great range of optimised formulations of the treatments for ADHD out there, and rather than guessing yourself, it'll be better to have an honest conversation with your doctor about the ability to mix XR/IR in a way that works for you.

This is correct. Concerta is interesting because it's a laser-drilled time release. Vyvanse is interesting because it's a prodrug of amphetamine that releases chemically over time.

The XR part is usually the particles inside the capsule. To make them faster acting you also have to pulverize those.