"I don't have to remind either of them that I'm still quicker than them, they know," 37-year-old Blythe says with a smile, making sure both sprinters are within earshot at Tauranga Domain this week.
"Joseph reminds himself often enough what my best time is. That was the catalyst for him asking me to train him - he wanted that national record."
Both sprinters head to Auckland in peak shape, with Harman the national 100m-200m schoolboy champion last year and winner of four golds on the sand at last weekend's national surf lifesaving championships.
Millar was buoyed by winning the 100m senior men's title at the Auckland championships two weeks ago in a wind-assisted 10.30s after spending 18 months battling a potentially career-ending back injury as well as low iron levels.
Blythe said both sprinters head to Auckland carrying sizeable targets.
"I'd expect Joseph to win the 100m and potentially the 200m as well and Kodi should win the (M20) double.
"Kodi's the one to beat in his age group whereas Joseph has a bit more competition but beat his two main threats [Isaac Tatoa and Carl Van der Speck] a couple of weeks ago."
Blythe won the New Zealand senior 200m title but lived in Gus Nketia's shadow on the shorter stretch of track.
He counts the world championships in Stuttgart in '93 as his career highlight, retiring three years later and switching to rugby, where he playing a couple of dozen games for the Bay of Plenty Steamers.
"I'd hardly been to a track in 15 years until Joseph called and certainly had no coaching experience.
"I wondered going into it how much things had changed, but apart from having a decent track to train on now, sprinting's still sprinting.
"Getting that good winter base and building strength into them is the key.
"They're both young and naturally fast but as you get older you need to develop your muscles more, although I've had to balance that with the back issues Joseph has had."
Millar injured his lower spine 16 months ago and was at a low ebb last year, losing both M19 sprint titles. Frustrated by a lack of improvement with his injury he ignored Athletics NZ's high performance treatment plan and sought help from Dr Pat Wigley at the CNS (physio and chiropractic) Clinic.
"A month after I started seeing him there were good improvements and then it all just clicked.
"All I wanted for my birthday [last September] was to be pain free and have my legs back. I still have to manage it but I'm past the point where it's affecting my running."
Harman is firing, winning the 100m in 10.67s and the 200m in 21.39s, a new national record, at the secondary schools national last year. He is in the national surf lifesaving squad for the world champs and Blythe likes what he sees in the teenage whippet.
Following on from the nationals the pair hope to go to the Australian champs from April 13-15 to further their quest for Olympic qualification times - 10.21s for 100m and 20.59s for 200m, the national sprint squad also lining up a possible tilt at a M20 4 x 100m relay record.