You can expose the service in two different endpoints. the SOAP one can use the binding that support SOAP e.g. basicHttpBinding, the RESTful one can use the webHttpBinding. I assume your REST service will be in JSON, in that case, you need to configure the two endpoints with the following behaviour configuration
An example of endpoint configuration in your scenario is
so, the service will be available at
<a href="http://www.example.com/soap">http://www.example.com/soap</a>
<a href="http://www.example.com/json">http://www.example.com/json</a>
Apply [WebGet] to the operation contract to make it RESTful. e.g.
Note, if the REST service is not in JSON, parameters of the operations can not contain complex type.
For plain old XML as return format, this is an example that would work both for SOAP and XML
POX behavior for REST Plain Old XML
Endpoints
Service will be available at
<a href="http://www.example.com/xml">http://www.example.com/xml</a>
REST request try it in browser,
<a href="http://www.example.com/xml/accounts/A123">http://www.example.com/xml/accounts/A123</a>
SOAP request client endpoint configuration for SOAP service after adding the service reference,
in C#
Another way of doing it is to expose two different service contract and each one with specific configuration. This may generate some duplicates at code level, however at the end of the day, you want to make it working.