In BrE:
Multi ---> Mul-tee
Multitask ---> Mul-tee-task
AmE:
Multi ---> Mul-teye
Multitask ---> Mul-teye-task, sometimes Mul-tee-task.
Similarly:
BrE:
Anti ---> An-tee
Antigravity ---> An-tee-gravity
AmE:
Anti ---> An-teye
Antigravity ---> An-teye-gravity