If your Date and your Time fields always have data, your new date time field could use a concat formula for its default value: concat(datefield, "T", timefield)
If they don't both always have data, you may want to use rules for this instead. Add a rule on the date field to set the new date time field using a formula like the one above, if time has a value. If time doesn't have a value, you could set the date time field to blank, or use: concat(datefield, "T00:00:00") for a blank time. On the time field, you'd also need rules to set the new date time field. You may also need rules in the Form Load rules to set the new field according to whatever logic you need (that is, whatever you decide should happen if one of the two fields has no data).