Looking over the plumbing fittings in the hardware store today, I saw a 1/2 X 1/2 X 3/4 tee copper capillary (lead-free) fitting, and I finally succumbed and bought two. (one half inch joint was the side leg) They had 2 ft lengths of 1/2 copper pipe and 3/4 inch copper pipe which I bought too, and I thought there was a good chance of removing the upset shoulder of the tee which is intended to prevent a pipe entering too deep. And so it turned out, after some rat-tail filing.
When I cut off six inches from the three quarters pipe, I ended up with a half inch pipe with straight ends inside a 3/4 inch pipe that was fed by two side 1/2 inch inlets.
As you can tell, the gross cross section of the 1/2 inch pipe and the
1/2 to 3/4 inch annulus is reasonably balanced, though the annulus has twice as much surface friction to cope with from its inner and outer wall.
But for pocket change, I can now run a reasonable trial of a reverse flow specimen, good for laughs.
Brian W