There's really only one act in the current known history of the Cosmere that was done by the combined power of two or more Shards and hasn't been done since:
Creating a planet, literally from scratch.
We are aware that general planetary engineering - moving a planet's orbit around a star, or terraforming it to make it livable - is something that a single Shard is capable of, as we see that directly in the Mistborn novels. During Hero of Ages, Vin takes up Preservation and tries to move the planet around to fix things. Possibly this may be influenced by a Shards intent, but it is likely something all Shards have the power for, given how quickly and instinctively she was able to start doing it (even if her results were not as successful). The number of planets with active Shards that have living humans in natural environments reinforces this assumption. The number of star systems with unusual arrangements (a tidally locked planet that is still livable, the strange arrangements of planets in the Rosharan system) practically confirms that moving and changing planetary bodies is old hat to anyone with god-level powers.
What we haven't seen at all is an event like the creation of Scadrial. Per Arcanum Unbounded, the planet Scadrial on which the Mistborn books take place did not exist at all before Ruin and Preservation decided to start their work there. No other Cosmere world carries that distinction, even in systems where it's feels like there's been heavy stellar tinkering. The uniqueness of it leads me to conclude that a creation on that scale could only have been done by two Shards working in concert.
This of course leads to unfortunate questions, such as what might happen to that planet should the Shards that are so strongly Invested in it be removed.
Beyond this, we only really have speculation.
No general overview of the limits of Shard power has ever been given in any out-of-universe Q&A that I can find. If Khriss has managed an in-universe scientific analysis of those power levels, we haven't yet been granted access to it. If the limits of being a god in the Cosmere are basically that any given planet is your toy, we have a fairly high bar to leap to find things that might fit the mold of multiple Shards. Then again, there's also the smaller scope to consider - Khriss' interests are in how Investiture manifests into magics and powers and the variances, and we have only a few ideas of how this works too. This is also not accounting for events that Shards don't seem to completely control, that look like a side effect of the sheer amount of Investiture they represent, like perpendicularities and the formation of Shardpools.
So to throw out the ocean of possibilities -
- Stellar manipulation and engineering. Ringworlds. Singularities? The Culture level sci-fi stuff.
- Magical oversaturation of a given setting, where literally anyone has easy access to physics-breaking powers.
- Magical overcomplication. Single Shards tend to invest and create specific systems of magic, whether intentional or not. We've already seen how two Shards interacting created three magic systems on Scadrial. We're actually not even sure yet what sort of magical interaction is going on with Roshar, where three Shards interacted, and some of it is older than humanity.
- If perpendicularities are what happens at a concentration of Investiture - essentially enough magic flow to punch through and link all the realms at one point, something that naturally happens with one Shard - this implies a much heavier cross-realm merge is possible with a stronger concentration, say, that of all sixteen Shards (we have yet to understand why Adonalsium was shattered, that may be one of many reasons)
Finally, there's also a flipside to phenomenal cosmic power. We know that the act of Shard's tinkering with worlds is essentially a form of Investment, that part of the Shard's power becomes part of that world. It has been confirmed that the more this goes on, the harder it is (not impossible, just not effortless or instant) for that Shard to gather it's power up, pack up, and move somewhere else (thus why Scadrial's fate is so tied to that of it's Shards). Combining more Shards would logically double down on this effect, limiting the mobility of any would-be god that grew more powerful than his fellows.