Maybe Splinter Cell is my only real legit example, but still I thought SC:CT on the ps2 looked better than many exclusives for the system.
I guess the point I was trying to make is just because a game is multi-platform, that doesn't mean it can't push a system.
Well it doesnt mean it happens always
The problem is, if two platforms are somewhat similarly powerful but not the same (i.e different strengths and weaknesses) you will see multiplatform titles that may push one platform more than the other or push both in areas that dont demonstrate their specific strengths. They will push them in the areas that both can do well
It becomes a bigger problem when one has a kind of "unfamiliar" architecture and demands some special care to get something out of it. In such cases they may design the game based on the platfom that fits best what they want to do and simply port it to the other platform (which may end up looking worse), design it based on the lowest common denominator to ensure identical quality, or design the game based on the common capabilities and optimize it as much as possible for both.
What you wont see happening is designing a project with completely different directions for each platform.
The only and closest example I can think of is Ninja Gaiden 2 vs Ninja Gaiden Sigma 2 which were developed at different time frames by different teams. Sigma 2 was redeveloped for PS3's capabilities whereas the original was more fitting to the 360's capabilities. They pushed the platforms from different directions
In general multiplatform titles will more or less target the same visual characteristics and will look and perform similarly in identical sitiuations for both platforms.
Check Castlevania. I am pretty sure it pushes both. But I doubt it exploit's each of the two platforms unique capabilities. It relies on what both can do well.
While God of War 3 pushes the PS3 in different ways than what a multiplatform title would. Same may count for Uncharted 2 and Killzone 2. Those games were designed while keeping in mind what situations the PS3 performs best.
Gears of War 3 appears to be a similar example for the 360 although its running on UE3.
The unified memory and edram are supposed to allow for some unique stuff that the PS3 doesnt allow. So why not make a game that uses them to their full advantage?