Teoreticaly, divide file to say 5 segments and login 5x and ... hey, it should work, but what is the point?
If the server is slow, the it should be upgraded. You connection limit can't be broken either and above all that, most sites limit the number of ftp connections to one or two
Not to mention that this is quite hard to implement with possibly minimal gain
PS. don't steal more time from the server for just you, off the other users
