I am no expert in pneumatics (I had to make sure I was spelling it right when I typed it just now) but I have a thought.
The demo models in the product generally seem to use Constant Volume Chambers to represent pipes, and I do not see too many cases of orifices being directly connected in series.
I wonder, if you add a Constant Volume Chamber between the two orifices, and then just make the volume of the chamber very small (so it represents the pipe between the orifices), does that allow the system to converge? I gave it a shot on my computer (with made-up parameters - again, I know nothing about pneumatics) and it seemed to work.
As far as "missing something fundamental", my understanding of why this problem may have occurred is that both orifice blocks do not seem to contain any state - instead they provide direct algebraic relationships between the inputs/outputs. So, as you chain them together, you end up with some fairly complex equations that Simscape has to solve. When you introduce a Constant Volume Chamber, which contains a state (the chamber pressure), Simscape can more easily solve the equations since the orifice's algebraic relationships are not chained together directly. Again, I'm not an expert so please take this interpretation with a grain of salt. I'd be very happy to have someone set me straight with a clearer interpretation :)