When I built my cargo bike, I was scrambling for parts, and ended up changing plans on configuring the gearing. Somehow I managed to get different crank lengths which I rode for a while and didn't really notice it, although my legs were always a bit sore, which I attributed to the wide bottom bracket. So, you can certainly ride a bike with different crank lengths.

How big of a difference are you thinking about?

Changing the crank length will change a few things with your stroke. The longer cranks means the pedal moves further away from the seat at the bottom of the stroke, and you typically lower the seat. But, it also moves closer to the seat at the top of the stroke, more bending the leg.

The circumference of the circle also increases as well as leverage changes. In effect, one leg would be moving at a higher cadence, and the other would have more power.

Anyway, it all seems like it would be a suboptimal solution, except perhaps in the most extreme conditions.

Are you using SPD or SPD-SL? I think I've seen SPD pedals that are different thicknesses (different brands). They might vary up to a couple of millimeters. It might be worth browsing around a bit, and changing the pedals might be as good as changing the cleats. Plus, if you walk on the shoes, you won't have the cleat sticking out and rubbing on the ground on one side.

With some luck, a bike shop might have some orphan pedals.