I did this recently without a helper. It was easier to get the heads off than to put them back on. If I do this again I will try to use an engine crane to levitate the head over its final position. Otherwise you risk damaging your new head gaskets.
It is significantly easier to do this type of work on an engine stand. Doing it in the car was difficult and I was not fully prepared for the effort needed to reinstall the heads. The heads are heavy and awkward.