Can I replace an electric range with a cooktop and separate oven?
As long as the ampacity of the existing 220VAC branch circuit is sufficient to support the new cooking equipment, you can do this, despite it not being called out explicitly as allowed for by the NEC.  The closest cites I can find in the Code for this are 210.19(A)(3) Exception 1:


  Exception No.1: Conductors tapped from a 50-ampere branch circuit supplying 
  electric ranges, wall-mounted electric ovens, and counter-mounted electric 
  cooking units shall have an ampacity of not less than 20 amperes and shall be 
  sufficient for the load to be served. These tap conductors include any 
  conductors that are a part of the leads supplied with the appliance that are 
  smaller than the branch-circuit conductors. The taps shall not be longer 
  than necessary for servicing the appliance.


210.23(C):


  40- and 50-Ampere Branch Circuits. A 40- or 50-ampere branch circuit shall 
  be permitted to supply cooking appliances that are fastened in place in any 
  occupancy. In other than dwelling units, such circuits shall be permitted to 
  supply fixed lighting units with heavy-duty lampholders, infrared heating 
  units, or other utilization equipment.


and Table 220.52, Notes 3 and 4 (reformatted to use decimals instead of improper fractions for kilowattage):


  
  Over 1.75 kW through 8.75 kW. In lieu of the method provided in Column C, 
  it shall be permissible to add the nameplate ratings of all household cooking 
  appliances rated more than 1.75 kW but not more than 8.75 kW and multiply the 
  sum by the demand factors specified in Column A or Column B for the given 
  number of appliances. Where the rating of cooking appliances falls under both 
  Column A and Column B, the demand factors for each column shall be applied to 
  the appliances for that column, and the results added together.
  Branch-Circuit Load. It shall be permissible to calculate the branch-
  circuit load for one range in accordance with Table 220.55. The branch-circuit 
  load for one wall-mounted oven or one counter-mounted cooking unit shall be 
  the nameplate rating of the appliance. The branch-circuit load for a counter-
  mounted cooking unit and not more than two wall-mounted ovens, all supplied 
  from a single branch circuit and located in the same room, shall be calculated 
  by adding the nameplate rating of the individual appliances and treating this 
  total as equivalent to one range.