...2 or 4?
Doubling the snare drum back beat with a single strike on the tambourine is quite common and can give some variety to the groove. I usually choose one backbeat or the other; seldom do I play both 2 and 4. Less tedium and greater variety occurs when the snare drum is heard alone on one of the beats and the doubled sound on the other. Sometimes my choice is arbitrary but often I make a studied decision based on the lyrics of the song.
Listen to the phrasing of the lyrics and you will often find less vocal activity around either beat 2 or beat 4. Playing on that back beat allows the tambourine’s tone color to do its job without competing with the vocals.
Both lyric and tambourine get their space.