Altium : How to design a width rule for a subset of a net?
Three approaches:

Use a net-tie: A net tie will create two distinct nets (each has its own name) but they are actually joined. Information on how to create &amp; use a net tie can be found here - Altium - Using Net Ties to Meet PCB Design Requirements.
Use a 0R resistor. Again, this will create an additional net.
Scope your rules so that they target only specific areas. You can either target your rule to only apply within a specific room or within specific coordinates on your board.