I'm with untamed on this, if your primaries were Tee'd see no reason why a single valve wouldn't work.
My self have thought about converting my 2 overflows quite abit to a Herbie with one overflow being primary and the other overflow chamber being the secondary (single pipe in each). If the overflow weirs the same height would just be filling the second chamber, but would work if raised the height of the weir on the second chamber, thinking less than 1/2".
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