Let me start by saying that Rommel (nor anyone else for that matter) couldn't have supplied two more divisions as far forward as Alamein given the original logistical position of the Germans. Rommel was having great difficulty getting supplies forward for those troops he already had there. This, in a nutshell, is the German problem in N. Africa throughout this campaign, a lack of logistical and engineering support and planning. For instance, neither Bengahazi or Tobruck harbors were cleared or repaired by the Germans. This limited these two ports to just one or two ships at a time unloading. Ships couldn't anchor out and unload as the Germans (nor Italians) ever brought over any lighters or barges for this purpose.
The lack of a rail line across Libya was also a major problem. If anything, Rommel needed several thousand Organization Todt workers and railroad engineers with proper equipment to lay track. This would have alleviated most of his supply problems in itself.
Of course, if anything, the Germans were very poorly equipped and lacked good management and planning skills in the area of infrastructure and civil engineering. This led to massive wastage on every front, not just N. Africa (several thousand workers with nothing but picks and shovels don't accomplish much quickly).
The reason far larger units could be supported in Tunisa was that there there were several large ports and, the French had developed more infrastructure that could support a larger army in the field.