"We started off very well in the first Twenty20 and to back it up with a batting and bowling performance like that is very pleasing.''
Zimbabwe won the toss and put New Zealand in to bat this morning and McCullum and Guptill quickly went about peppering the home side's bowling attack to all corners of the Harare Sports Club.
McCullum, who made an unbeaten 81 in the first game, survived a confident lbw appeal early on to smash 64 from 37 balls and, like his effort during the weekend, his knock included six sixes.
In pursuit of more than nine runs per over, the home side batted with plenty of intent as 25-year-old opener Chamu Chibhabha carved an impressive 65 from 39 balls and he found good support in Elton Chigumbura, who made 39 from 22 deliveries.
But if Brendon McCullum was impressive with the bat, his older brother Nathan took his chance to shine with the ball.
The 31-year-old from Otago took three wickets, including the scalps of Chibhabha and Chigumbura, to ensure Zimbabwe wouldn't threaten an unlikely victory and were bundled out in the last over for 154.
Rookie all-rounder Doug Bracewell also claimed three wickets, while evergreen Northern Districts seamer Graeme Aldridge made his full international debut for New Zealand at 33 years and 336 days old.
It wasn't exactly a day to remember statistically as the right-armer saw his four overs disappear for 45 runs, although he did bag one wicket and a catch.
After numerous tours with the national A side and years spent on the fringe of the Black Caps, Aldridge wondered if his chance would ever come.
"There's probably been lots of times when that has been my thoughts,'' he said. "But my plan has always been to play well for ND and what will come, will come. It's a good day. [I've] obviously been working away at it for a while, but it was a pretty good moment for me.''
Both sides now switch their focus to the 50-over format with three one-dayers awaiting them.
The first starts in Harare on Thursday night (NZT), with the second two days later and the third in Bulawayo on October 25.
Guptill said the Twenty20 clean sweep was a good start to the tour, which also includes a one-off test match.
"It's given us some good momentum going forward for the rest of the series,'' Guptill said.