Well, that's not so easy but not impossible.
I made a sample for you.
; Reference to the date/time fields
var dateTime1 = Ref(DateTimeField1)
var dateTime2 = Ref(DateTimeField2)
; Extract the dates from the fields formatted values and convert them into numbers
var date1 = Date2Num(Left(dateTime1.formattedValue, At(dateTime1.formattedValue, " ")-1), "M/DD/YY")
var date2 = Date2Num(Left(dateTime2.formattedValue, At(dateTime2.formattedValue, " ")-1), "M/DD/YY")
; Extract the times from the fields formatted values and convert them into numbers
var time1 = Time2Num(Right(dateTime1.formattedValue, At(dateTime1.formattedValue, " ")-4), "H:MM", $.locale)
var time2 = Time2Num(Right(dateTime2.formattedValue, At(dateTime2.formattedValue, " ")-4), "H:MM", $.locale)
; Get difference in minutes
var dateMinutes = (date2 - date1) * 1440
var timeMinutes = Abs(time2 - time1) / 60000
; Show result in numeric field. Use display pattern num{z,zzz,zzz,zzz,zz9.88 'Minutes'}
$.rawValue = Sum(timeMinutes, dateMinutes)