Of course you can use the same wetsuit. Water is water, cold is cold, neoprene is neoprene. The only conceivable reason I can see for buying two is if you decide one shortie wetsuit is warmer than another, and therefore more suitable for, say, kayaking.
Farmer John Short-Leg
You could, for instance, get a Warmers 2mm AquaLite Short-Leg Wetsuit ($69; available at REI.com), a good base layer adequate for more temperate aquatic sports. When things get chillier, add a Mysterioso Nyloprene Top ($59; www.mysterioso.com), which combines neoprene with stretchy nylon panels in a light, warm shirt-type top.
In any event, there are lots of options. Some people prefer the extra warmth of a long-legged sleeveless suit, such as the Warmers 3mm Titanium Farmer John (REI has these for $119), then add a short- or long-sleeve neoprene top/shirt. Or, get the shortie suit and have in your arsenal a long-sleeve neoprene top, such as one made by Zap ($60; available at www.skimcity.com). Point is, you can spend maybe $200 and have three pieces that will suffice for a variety of conditions.