You need to forget the negative entirely - leave it out. You want to consolidate two fields - from left to right. That's very easy.

 sed '	s/ *\(.*\)/\1 /
			s/\([0-9]* *\)\{2\}/\1/g
			s/ *$//
	' <<\IN
 -1 2 3 4 -5 9
 2 3 -4 5 -6 11
 IN
 -2 4 -9
 3 -5 -11

Notice how I avoid any reference to the sign at all - when the input is processed the automaton will *accept* only spaces or numbers because it understands nothing else - all else is ignored completely and will remain in place.

When you specify a `\{`numeric repetition interval`\}` for a `\(`subexpression`\)`, only the last occurrence of that expression is `\1` back-referenced. So you can just squeeze - or truncate - a repeat interval that easily. And because we squeeze the repeat behind the sign - if there is one - the second occurrence of that pattern will follow any sign that used to precede the first.

The behavior described above is specified by POSIX for *all* BRE compliant applications, but very few `sed`s get it right. GNU `sed` does.

Last, the spaces are just to make the pattern occurrence *regular*.

Of course, this will never work for you. Or, probably more correctly, it will *always* work for you, but never return any results. How could it if the pattern is *indefinite*?