Oxford gives three usages for this construct:
不...不...
bù...bù...
① [used to make an emphatic negative form of two words identical or similar in meaning]
不骄不躁
not conceited or rash, free from arrogance and rashness
不慌不忙
unhurried, calm, leisurely
不知不觉
unawares
② [used with two words of opposite meanings to indicate an intermediate state]
不大不小
neither too big nor too small, just right
不冷不热
neither cold nor hot
不死不活
neither dead nor alive, half-dead
③ [the first 不 is the condition of the second 不]
不见不散
We won't leave until we meet
Collins also gives three, pretty identical, definitions:
不 ... 不 ...
(bù ... bù ... )
1 (表示强式否定)
" ⇒ 不知不觉" (bù zhī bù jué) "unwittingly"
" ⇒ 不慌不忙" (bù huāng bù máng) "unhurriedly"
" ⇒ 不折不扣" (bù zhé bù kòu) "one hundred percent"
2(= 既不 ... 也不 ... ) "neither ... nor ... "
" ⇒ 不亏不盈" (bù kuī bù yíng) "neither too little nor too much"
" ⇒ 不大不小" (bù dà bù xiǎo) "just the right size"
" ⇒ 不死不活" (bù sǐ bù huó) "half dead"
3(= 如果不 ... 就不 ... ) "unless ... then not"
" ⇒ 不破不立" (bù pò bù lì) "without destruction there can be no construction"
" ⇒ 不见不散。" (Bù jiàn bù sàn.) "See you there."