The Junior Warriors rose to 2-0 following a shock straight-set 25-20, 25-18, 25-22 conquest of the Bullpups.
Team captain Francis Babon topscored for UE with 12 points, 11 receptions and nine digs while Giles Torres and Joshua Espenida each had 10 hits.
Michaelo Buddin led NSNU with 12 points.
UST was able to utilize all of its players and had three players in double digits led by Rey De Vega's 13 points.
"Nagexperiment kami with different combinations so as to give other players exposure para in the future kailangan silang gamitin, they are ready," said assistant coach John Abella after the Junior Tiger Spikers followed up last Wednesday's season-opening win over the Bullpups.
Ateneo, which was paced by Alexis Mendoza with 10 points, fell in a tie with NSNU at 0-2.
Ahead by just two points in the fourth set, 8-6, UST needed a 17-6 blast, punctuated by CJ Segui's off the block kill to secure the second victory.
Emmanuel Advincula registered 16 points built on 12 spikes and four service aces while Angelo Lipata and France Racaza added 13 apiece for the Baby Tamaraws, who dominated the spiking department over the Junior Green Spikers, 64-50.
Action resumes on Wednesday with three girls' games and one boys' tilt at the Paco Arena.