If you have sufficient disk space, it's a good idea to install mohaa another time before you install spearhead. In the end you will have a separate mohaa and a spearhead folder, so you can be sure nothing will be messed up.
If you haven't installed Spearhead yet, you'd best install another Mohaa in an other folder. So for example have a mohaa installation in a /mohaa folder and another one in a /mohsh folder. Then install Spearhead in the /mohsh folder.
Very simple....move the two spearhead pk3 files out of the mainta folder (but dont move then into the main)....do your mappin and complieing....then move them back in to play spearhead.