Hi,
Well for a given point in space if they have the same field there then they have the same 'strength' there. But when we have different shapes it does get a little tricky because we have to take every point inside the magnet into account, and that means that as an object is approaching the front face of the magnet a different thickness magnet will have a slightly different effect on all the points of the object. So we have to at least specify a distance for the object. The strength is measured as the pull force, and as an identical object nears the front face of both magnets we'll see different forces, and that's because a larger part of the thicker magnet is 'behind' the front face while most of the thinner magnet mass is very close to the face.
So an approximate model would be to model the thin magnet as a single thin surface with some given strength as measured at a known distance, and model the thick magnet as two thin surfaces one slightly behind the other. In order for both composite 'magnets' to have the same strength at a given point out in front of them, the two surface magnet would have to have a material that is not as magnetized as the single surface magnet. That's because the two surfaces 'strengths' add up at the point of interest differently.
For example, lets say that at a distance of 1 inch we have equal force at a point on the object with both the single surface magnet and the dual surface magnet. If we then move the object closer to the single magnet we see a change in force of about 1.235, while we see a change in force of the two surface magnet of about 1.223 (same small distance moved about 0.1 inch). Not much difference, but now the single surface magnet looks stronger when before they exerted the same force on the object. We again assumed that the magnetization of the dual magnet is less than the single magnet as that is how we got them both to exert a pull force that was equal at a distance of 1 inch.
But if in a given application if both magnets exert the same force, then they have the same strength.
Did you have some specific reason you needed this information?