Not sure if you are asking if it can be paired with a second monitor (it can) or if you are asking for monitor recommendations. If the latter, just about any monitor can be used. There are so many possibilities that if you want recommendations, you need to add some details in what you want. Bigger? Smaller? Wider? Could an iPad suffice as an extension monitor? If you only need temporary uses, could perhaps linking to a TV cover the need?
Note too that this is a biased crowd, so you are going to get a bunch of recommendations for the ASD because that's the only consumer monitor Apple makes. So if you end up with 80% push for ASD, understand that you asked Apple people for a monitor recommendation. If you asked Dell people for a monitor recommendation, the percentages would likely be as dramatic (yet very different).
Share detail on WHY you want a second monitor. And bigger or just a little more screen? And if you are trying to get something that visually "matches" or you don't care about that. Etc.
