How to parse a line that is delimited by a colon but sometimes has two colons?

[Costello, Roger L.]

Hello DFDL experts!

I am using DFDL to parse lines that look like this:

name: value

I am using this DFDL code to parse the lines:

<xs:sequence dfdl:separator=":" dfdl:separatorPosition="infix">
    <xs:element name="name" type="xs:string" />
    <xs:element name="value" type="xs:string" />
</xs:sequence>

If the value is base64 text, then a double colon is used:

name:: base64-value

The above DFDL code doesn't seem to work in this situation. What's the correct 
way to write DFDL code which can handle lines with a single colon as well as 
lines with a double colon?

/Roger