Let me see if I understand you correctly. You have two fields with IF formulas, each generating some output. Let’s say that Field 1’s IF output is “First”, and Field 2’s IF output is “Second.” In another field you want to combine them both so that it displays something like “First Second”. Is that correct?
The in-app annotation help for ARRAYJOIN says that it “Joins the array of rollup items,” which doesn’t apply to this case. However, this works:
{Field 1} & " " & {Field 2}
Obviously change the " " contents depending on how you want the two pieces joined.