Hi Comer,
that's not really something you'd expect to be built in, but the hooks are there to apply pretty much any scripting language to get the job done: for example, using Perl.
(In Textmate)
Text => Filter through command...
perl -pe 's/\n/ / if ($.%2)'
This substitutes a space for the newline character at the end of the line if the line number ($.) is an odd number.
Cheers, Paul