I'm not sure I understand your question. "Meantime" can be a single word in English, but I believe (although I'm not 100% sure) that leaving it as two words is also considered to be OK. English is a messed up language, but many of the compound words (like meantime, meanwhile, anytime, etc.) started out as two words, and then at some point became joined into one word. Some of these are considered proper grammar as either one word or two words. Others are generally considered to be improper grammar if split into two words. And of course, in order for them to be proper grammar as one word, the combination has at some point in the past to have become accepted, so if you're not a native English speaker (and perhaps even if you are), it can be difficult to know when two words should be joined together into one, and when they shouldn't.
Some of the more messed up of these are words that begin with "al" like already, although, and perhaps best of all, albeit (a contraction of "all be it"), since one of the l's is dropped from the original "all". Additionally, they can have a meaning as a compound word that differs from the meaning when used as two words. "Already" means that something has happened in the past (The sun has already risen.), while "all ready" means that everything is ready. (We're all ready to go now.) It's perfectly legitimate to use "all ready" as two words with the latter definition, but probably not with the former, and "already" can never mean "everything is ready".
Some of the more messed up of these are words that begin with "al" like already, although, and perhaps best of all, albeit (a contraction of "all be it"), since one of the l's is dropped from the original "all". Additionally, they can have a meaning as a compound word that differs from the meaning when used as two words. "Already" means that something has happened in the past (The sun has already risen.), while "all ready" means that everything is ready. (We're all ready to go now.) It's perfectly legitimate to use "all ready" as two words with the latter definition, but probably not with the former, and "already" can never mean "everything is ready".