Do Shimano SL Pedals = LOOK Pedals?

You're correct: They aren't the same

You're better off replacing 1 set of pedals and keeping 1 set of cleats and shoes for both bikes. Swapping cleats on a pair of shoes to ride Bike A instead of Bike B is a bad idea as positioning the cleats is part of the bike fit process (or should be).

Change out one of your sets of pedals so they're both the same and make sure to use the cleats for the pedals you have chosen.

Then sell off your off-brand pedals and recoup part of the price for the new set.

Shimano SLs started out as LOOK copies - Shimano licensed the technology from the LOOK company.  But things tend to diverge and change over time.

I think what you're really asking is "can I use my current Shimano SL cleats in a 10 year old LOOK pedal", and that may be hard to answer.  LOOK has had at least a couple different types of pedals and cleats with small differences.  I recall buying the wrong LOOK cleats once and had to return them - I bought black ones, and I needed red ones.   The there was LOOK "Keo' pedals, which I don't know the difference.   Perhaps someone else does.

Your best bet may be find someone with LOOK cleats and see if they just fit, rather than buy a set yourself.  Maybe your LBS can help.

No, they are not compatible. Cleats should be considered a wear part and replaced about once a year (depending on milage, but more importantly, wear... if you have to unclip often, obviously the cleat will wear more, if you walk in the them without cleat covers, they will wear quicker and so on). A worn cleat can result in unintentional unclipping, which often results in a crash... With the fairly low price of a set of cleats, replacing them a bit more often isn't the end of the world...

The different colors for Look Cleats have to do with degrees of float and not what pedal they work with.   Red is 9 degrees, Grey is 4.5 and black is 0 (or fixed).

Look KEO cleats only fit Look KEO pedals.  If you have the older style Look pedal, you need different cleats.  This will not work with Shimano pedals either.

If you post a picture of your pedals, someone can tell you what they are and what cleats.  Your LBS can do this also.