Not sure if it works in straight
vi, but I can do it in
vim. Just use the
block selection tool (ctrl+v), to highlight and 'd' the second block. Then move up to the end of the first line and simply 'p' paste it.
Of course this depends on there being two blocks of text that line up. If you're actually trying to do something fancier like concatenate line n with line n+m, then you'll have to do something like record a macro or write a script.
http://vim.wikia.com is a good place for looking up vim how-to stuff.
