Usually, having multiple sources of thrust that aren’t coordinated with each other is bad news. If the user managed to keep their body completely straight and still, I would imagine that the combination could give them extra speed. However, rocket boots are more maneuverable than jetpacks and much more unstable, so I think it’s more likely that they would end up interfering with the jetpack’s ability to maneuver and cause the user to go all over the place, because those rockets have the potential to point in any direction if the user can’t control them perfectly. 

Hope that helps!

~ Jacen