arrayName.concat({item1{, item2{, . . . {, itemN}}}})
arrayName.concat({item1{, item2{, . . . {, itemN}}}})concat returns a new array consisting of a combination of arrayName and any other supplied items. The items to be added (item1,...,itemN) to the array are added, in order, from left to right. If one of the items is an array, its contents are added to the end of arrayName. If the item is anything other than an array, it is added to the end of the array as a single array element.
Combining arrays with codes answered on two questions into one array
If you have two multi questions and would like to create an array consisting of all the codes answered on the two multi questions, one possible way is to combine the two arrays you get from the categories method:
var q8 = f('q8').categories();
var q9 = f('q9').categories();
var codes = q8.concat(q9);