McDonald is in his first full season with the Warriors having previously spent time in the academy and the big lock forward has played regularly for Glasgow Hawks in the BT Premiership.
Since re-joining his hometown club, the 24-year-old has played 11 times this season including three appearances in the European Rugby Champions Cup.
26-year-old Tagive has played five times for Glasgow, scoring on his home debut against the Dragons last season.
The winger has featured for Currie Chieftains in the BT Premiership this season helping them finish second overall, reaching the league semi-finals.
Speaking to Warriors TV, McDonald said "I'm absolutely stoked. This season has been above and beyond my expectations. The opportunity to play against Montpellier was fantastic and my highlight was playing Edinburgh at home.
"Glasgow is such an amazing city and to be part of a close-knit team made it an easy decision for me to re-sign."
Tagive said: "I'm grateful to Ben (Cairns) and the boys at Currie for helping me get back fit and playing. They're a great bunch of lads down there and I enjoyed my time with them a lot.
"I'm excited for the future. I've loved every opportunity I've had in a Warriors jersey so far and hopefully if I work hard there will be plenty more to come."
Glasgow Warriors Assistant Coach Kenny Murray said: "Both Ratu and Kiran have taken their chances well season and the fact that they've both committed for two more years is great for our strength in depth going forward.
"Both are incredible athletes. Kiran has played on the biggest stage in Europe this year and more than held his own.
"Ratu has also really impressed us too, displaying his pace and power in the BT Premiership with Currie Chieftains and for us after working hard to recover from injury."