I have an exterior receptacle on my house, and it is currently taking up 2 circuits (similar to kitchen counter plugs). Is there any code/regulation that indicates exterior receptacles must be on multiwire branch circuits, or can I convert it to a single circuit? I know HOW to do it, I just can not find any details of whether it is a code requirement (I could only find references to GFCI and grounding for exterior plugs). We do not have any high power exterior appliances/devices, it is only used for a car block heater in winter.
A dual circuit (where you have one hot conductor on one outlet and another hot conductor on a second outlet and they share a neutral) may not be 'required' anywhere. This kind of circuit is called a 'multi wire branch circuit', if you're having trouble finding the right verbage to find out it's required in your locality.
It probably should be a GFCI outlet at any rate. It may or may not be code where you are, but it's just a good idea.
As far as I'm aware, there are no requirements for outdoor receptacle outlets to be multiwire branch circuits. However, there may be some other safety requirements you'll want to consider.
Ground-Fault Circuit Interrupter Protection
If you follow National Electrical Code (NEC), the 2008 version section 210.8(A)(3) requires that outdoor receptacle outlets be GFCI protected. Even if your local code does not require it, it still may be a good idea to at least consider GFCI protection.
Since the receptacle will likely be subjected to the elements, you'll also want to think about how to protect the receptacle from damage. NEC 2008 section 406.8 covers this in detail. If the receptacle is in a protected area; where it is not subjected to beating rain or water runoff, a cover that protects the receptacle when it is not in use (no plug attached) is required. If the receptacle is not protected (in a "wet" location), it should be protected by a cover that offers protection whether or not an attachment plug is inserted.
In my opinion, it's best to always protect outdoor receptacles with covers that offer protection weather or not an attachment plug is inserted. The difference in price between the two types in negligible, and installation time is the same. It just makes sense to offer better protection.
Codes vary based on location, check with local codes before doing any electrical work. If at any time you are not comfortable doing the work, please contact a local licensed Electrician.