I'm planning on building a frenzy barb and using dual lacerators. Lacerator is throwing class when using doulbe throw. But which mastery is utilized when using frenzy/melee skill? Maybe axe mastery (since lacerator is an axe)?

your intuition is correct: when using throwing axes/javelins/throwing knives in ranged mode, they get buffed by throw mastery. if you use them in melee, they get buffed by axe mastery/spear mastery/blade mastery (last one only for D2R, D2L has no mastery for daggers)

Learned something new about d2 25 years later

just tried it and its true: axe mastery boosts melee attacks(frenzy), throwing mastery boosts double throw