How to concat multiple columns by index?

There are different ways of uploading files to Dataiku. I presume you have simply dropped the files in the flow in which case Dataiku will put the files in a server's file system connection and load them in a dataset with an arrow pointing up, indicating they have been uploaded. There is however a better way of loading files in Dataiku. First go to the Dataiku Administration => Connections settings and create a new server's file system connection pointing the path to a folder of your choice in your system. Then go your flow click on the Dataset button and select Folder. Create a new folder in the new server's file system connection. Once the new folder is created and you are opening it from the flow you can add files to it. Either drag and drop from Windows Explorer (multiple files selected are allowed) or click on the Add File button. Having now added the files to the folder you can add the Files in Folder dataset as per my advice on the linked solution above.

This is a much better solution because you can now easily update the files in the physical folder in your file system and have Dataiku reload the data without having to modify the flow. Whereas with the manual upload you did you will need manually update each upload dataset to refresh the data.

Finally as I said before you can easily add a row number to your datasets using the Window visual recipe. Just set the order to whatever column you want to order on in the Window definitions section of the recipe and then click on Row Number on the Aggregations section. Run the recipe and you will have a dataset with row numbers. Then use the Join recipe to join via row number and set the Join type to Full Outer. If this doesn't work post screen shots of your flow and how you are setting your recipes and what you get an output which is not what you expect.