two different text layers in one auto-scaling textbox?

The easiest and most flexible approach for this is using my extension Pins & Boxes.

Note that Pins & Boxes creates expressions. So the rigs you create with it also work inside templates and mogrts and also on machines, where Pins & Boxes is not installed.

Just get sourceRectAtTime() for t, and check for larger values.

The larger values are the ones, you take.

So:

s=thisComp.layer("Main title");
t=thisComp.layer("Main text");

s_w=s.sourceRectAtTime().width;
s_h=s.sourceRectAtTime().height;

t_w=s.sourceRectAtTime().width;
t_h=s.sourceRectAtTime().height;

w = max(s_w, t_w)

h = max(s_h, t_h)

[w,h]

*Martin