Hi, it's Jon here, but I'm replying with my Ebor Bikes hat on.
Single valve springs have their place, but there are 2 widely appreciated reasons for fitting dual springs
1 There is one spring left if a spring breaks. This is important!
2 Dual springs that are designed to rub against each other essentially eliminates spring surge, which happens at specific rpms with a single spring and which is a cause of spring failure.
BSA knew this when they designed the dual coils for the DBD34 Gold Star engine. These springs rubbed together and the outer spring was variable rate, which also acts against spring surge. The Eddie Dow valve spring conversion for the alloy head A10 used DBD34 springs for that reason. In comparison with the standard Super Rocket springs, this conversion has a lower valve seat force when the valve is shut and a higher force at full lift, in common with modern practice. The Dow conversion also used alloy collars, which in combination with the smaller Goldie springs, reduced valve train inertia very substantially.
In the distant past, using this conversion with Terry's Goldie springs, I had an outer coil break but the bike ran as normal below 6,200 rpm, i.e. the inner coil alone kept the valve under control till then.
I have come across customers who have been running the Dow conversion ever since the 1960s.
The spring wire we use is one of the best modern specialist valve spring alloys and is to a considerably higher standard than was available years ago.
My preference is for double springs.