Technically. Yes. They can. The cards and cartridges aren't that different that it would prevent such a thing.
However... Back when Super FX was a "thing" TDP wasn't much of an issue, the chips only consumed a couple of watts, they could be cooled without active cooling, so it wasn't much of an issue placing them inside a small plastic box without air flow.
Fast forward to today... To accommodate a secondary chip with a sufficient level of performance to make a big enough impact to justify such an approach, just wouldn't be feasible in such a confined space without sufficient cooling.
Fixed function specific-task secondary chips are an entirely different matter though. Then it ends up being all about cost.
For instance you could have a chip in the games card with a special-purpose pipeline for texture decompression for megatexturing... But you won't ever get a second tegra GPU in such a small space.
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