Of the two, van der Dussen has kept a few times at the provincial level, but Miller could well be the first choice should someone need to step in mid-game if de Kock picks up an injury. Miller has done the job before, first for KwaZulu-Natal Inland Under-19s and once for Dolphins in a three-day game. He was also the South African team's keeper at the Hong Kong International Cricket Sixes tournament in October 2012.
So, it had been a gap of almost seven years when Miller took up the gloves during the second ODI against Sri Lanka at Centurion - de Kock had bruised a finger - and he has been repeatedly field tested in the subsequent games. During the first T20I on Tuesday night, it was Miller who took the field as wicketkeeper, with de Kock fielding at mid off and midwicket. De Kock very nearly taking a catch in the outfield too, and later took back the gloves for the Super Over.
I'm just trying to make sure the ball gets in the hands, and I'm trying to be as neat as possibleDAVID MILLER
While he had previously filled in at the end of games during this series, on Tuesday night Miller was behind the stumps and under pressure from ball one. When ball two, bowled by Dale Steyn, brought an edge and a regulation catch that was safely pouched, a bit of that pressure dissipated. Miller, however, appeared a little surprised and unsure how to celebrate.
"I'm not too sure, it's still a bit unknown to me," Miller said of his wicketkeeping experience. "I was surprised [by the catch second ball] and I wasn't sure how to celebrate. I'm just trying to make sure the ball gets in the hands, and I'm trying to be as neat as possible. It's a good challenge and it's good fun.
"I prepped pretty well to be honest, so it wasn't shock [to take the catch]. But a lot of excitement, emotion and all that. It's a new challenge that I've been asked to do, that I'm not going to turn down. Quite a relief, and I'm pretty happy with it. [When I threw the ball up to celebrate], the ball didn't end up in the right place. I wanted it to go straight up, but it ended up behind me. But anyway."
He wasn't done. In Imran Tahir's third over - the 13th of the Sri Lankan innings - Miller read a googly that had flummoxed Kamindu Mendis and pulled off a sharp stumping to get rid of Sri Lanka's top scorer. "I actually prefer standing up than back," Miller said of keeping to Tahir. "And I've played a lot of domestic cricket with him at the Dolphins and obviously numerous years with the Proteas, so I can pick him pretty well. I was very stoked with that. The stumping was a highlight for me."
Mark Boucher, the former South Africa wicketkeeper and current Titans coach, has worked with Miller at training sessions this season, while de Kock has also helped out wherever possible to get him prepared to keep wicket in a match situation. While he appeared slightly flat-footed at times, Miller's performance with the gloves was functionally flawless, and the preparation has clearly paid off.
"It's not a natural thing that I've done," Miller said. "I've been a keeper in the past, when I was growing up. But it's not professional cricket. I'm as comfortable as I can feel. But I've prepped pretty well. It's not something that they've just decided to stick me in as keeper and I haven't prepped. I've prepped pretty well. And the more I work on it, the more it's going to feel comfortable."
His captain Faf du Plessis didn't deny Miller's nerves ahead of the audition, but praised the stand-in's 'perfect day'.
"We're working with Dave behind the scenes to explore that option," du Plessis said. "He was nervous to do it in a full match, but what a way to start with that nick off Dale, and a perfect day behind the stumps."
When asked whether the trialling of Miller as a wicketkeeper meant he would be the back-up at the World Cup and de Kock would be the only specialist, du Plessis' response was typically forthright: "Bru, I don't know. I'm not a selector, I'm not anybody. I'm just a player."
Should de Kock pick up a major injury in England, South Africa would be able to draft in a new wicketkeeper to their squad according to the ICC's World Cup regulations. But if something were to happen to him during a game, the evidence suggests that Miller would be a more-than-handy back-up.
Liam Brickhill is ESPNcricinfo's South Africa correspondent