What do you mean by 'merge tool'? RoboHelp doesn't include its own merge tool.
Are you using source control? (Working of a server suggests so, at least I hope.) If so, make sure to make check outs exclusive for RoboHelp projects. That way you will avoid merging issues. If you want multiple check outs, you will have to use a third party merge tool or the source control system built in merge tool to resolve conflicts.