Is it ensure, insure, or assure?
word-forming element meaning "two, twice, double, doubly, once every two," etc., from Latin bi- "twice, double," from Old Latin dvi- (cognate with Sanskrit dvi-, Greek di-, Old English twi- "twice, double"), from PIE root *dwo- "two." Nativized from 16c. Occasionally bin- before vowels; this form originated in French, not Latin, and might be partly based on or influenced by Latin bini "twofold" (see binary).
bi- 1 or bin-
Both sides, parts, or directions: biconcave.
Containing twice the proportion of a specified chemical element or group necessary for stability: bicarbonate.
Containing two chemical atoms, radicals, or groups: biphenyl.
Variant of bio-.