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