Matmata is actually easier to take than Gabes. Sure, lower Gabes can be taken, but until Upper Gabes is too, it becomes a deathtrap for armor, infantry and aircrafts alike.
Matmata simply needs a concerted effort to take it - Even without ESAI, it is sometimes taken. Mareth is the toughy. Because its in the open and under mortar and artillery barrage, it tends to change hands easily.
In the end I still feel, the 2-pronged attack would at least bring out German oversight at either the Western or Eastern flank i.e Toujane or Mareth, and allow the Brits to sweep through that sector
Playing as Brit, I noted that, when one side is under pressure, usually Mareth, the Germans pull troops from other sectors to defend it, so the other flank becomes easy - IF the Brits would treat each unprejudiced
IF mateth does fall, taking Lower Gabes isn't too hard, but its costly and ultimately pointless moves, since it will be recapped and at a great cost, afterall, you still have all German tanks intact and the allied lines become thin, stretching from Mareth to Gabes with easy pickings for Germans coming from the Tebaga Gap.