How do I merge two collections together

First, standardize the attribute names. Fathom will not recognize, for example, height and ht as the same attribute.

Select all of the cases in one of the collections using the Select All command in the Edit menu. Choose Copy Cases from the Edit menu. Select the collection into which you wish to put the copied cases. Choose Paste Cases from the Edit menu.

If you want the cases in the merged collection to reflect which original collection they came from, then, before doing the above, you'll want to mark the cases in each collection. To do so, first make a new attribute. Give that attribute a formula, designating the collection (if the formula is non-numeric, enclose it in quotes, like "Section1"). Then choose Clear Formula from the Edit menu (this is important, because you want the values to be treated as though you had typed them in, rather than generating them by formula).

Then make an attribute in the second collection, giving it exactly the same name as in the first collection. Give the new attribute a formula (such as "Section2") and immediately delete (clear) the formula.

Then merge the collections, as described above. For each attribute in collection one that has an identically named attribute in collection two, Fathom will merge the data.