Tune the track length of a segment but not the entire net in Altium
Maybe you could place a net tie in your schematic either between U1 and U2 and place this net tie at the end of trace you want to do the length matching on.
When using a net tie both of the signals on either side gets their own net name even though they should be connected.

I haven't tried this myself yet, but give it a read.


  This is achieved by connecting the 2 nets through a Net Tie component. A Net Tie component is nothing more than a controlled short circuit. On the schematic, the Net Tie component has two (or more) pins, with each pin connected to the nets to be shorted. Note that the pins are not wired to each other on the schematic, but they are connected together within the PCB footprint.


Altium description