Since posting this, I have put together an expression that works for what I need it for!
I have a control slider that animates from 0 – 1 then 1 – 2. This slider has the following expression:
anim = value;
startval = thisComp.layer(“CONTROLS”).effect(“START”)(“Slider”);
endval = thisComp.layer(“CONTROLS”).effect(“END”)(“Slider”);
if (anim<1) {linear(anim,0,1,0,startval)}
else {linear(anim,1,2,startval,endval)}
Still interested in case there’s a more efficient way of doing this, so open to ideas 🙂