They weren't the same for the journeys I was taking. For example, travelling one-way on the weekend to Clapham Junction whilst avoiding zone 1 has changed like so:
Welwyn GC: £11.00 -> £6.20
Hatfield: £10.20 -> £6.20
Welham G: £9.00 -> £4.70
St Albans: £13.70 -> £5.80
Certainly in this case Hatfield is the loser. And the fact that it is cheaper to use two contactless cards, switching at Hadley Wood or Brookmans Park and waiting 30 minutes for the next train is pretty dumb. It is cheaper still if you transfer between New Southgate and Arnos Grove, but I appreciate this is due to Sadiq Khan's super-subsidised £1.50 tube fares.
The only split ticketing opportunities that ought to exist in the TfL network are when an 'avoid zone 1' fare is not available due to the route exceeding the minimum journey time.