Convert text value to numeric value

Yes, that's correct. So to tie them you need to get the index of the value in the first array and then use that some index in the second one.

This could be the custom calculation script of the "MonthNum" field:

var monthTexts = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];
var monthNumbers = ["01", "02", "03", "04", "05", "06", "07", "08", "09", "10", "11", "12"];
var monthText = this.getField("MonthText").valueAsString;
var i = monthTexts.indexOf(monthText);
if (i==-1) event.value = "";
else event.value = monthNumbers;

In this case, though, since the second array is just numbers then you don't actually need it. You can use this instead (as the last line of the code):

else event.value = util.printf("%02d", (i+1));