The double declutch is well worth learning for non syncro gears,first and second on a 2a. Mostly for going from third to second. Second to first is not so easy as the first gear is straight cut and going to it needs to be done at standstill or very low speed at least until you get the hang of matching the engine speed to the road speed for the gear you are in which will be slow anyway.
To double it is clutch in,out of third to neutral,pause the gear lever and clutch up,clutch in and select second,clutch up.This all happens in under two seconds.
You could also try the "blip change". Clutch in, gear lever to neutral pause and blip the throttle, select gear,clutch out. The blip gives a chance for the engine and gear speeds to match and engage. Also good to help worn syncro box's keep going some.
I had a Foden two stroke in the sixties with a Foden 12 speed (four on the floor 3 splits on the column) couldn't wait to get to work every morningtry a twin split eaton box its a mans box
I thought heel and toe was brake and accelerator... ie a way of blipping the throttle while braking at the same time.I used to be very good at double de-clutching and when I got the Series I was looking forward to showing my prowess. But, double declutching really works best when you "heel and toe" so you brake with your heel and dip the cluch with your toe. I've never been able to get the Series pedals adjusted to make it doable. Basic double de-cluthing is essenstially changeing into neutral than changeing again from neutral to the gear you want, In reality on a Series with 4/3 syncho its easier to pull up in 3rd then change into neutral. I do blip and double de-clutch for 3-2nd if the traffic suddenly speeds up just when I was stopping but I pretty much never change 2 to 1 on the move, its too much faff when I'm usually about to stop anyway. One of teh great things about an overdive is you get anotehr syncho gear so you can do quite lot with 3, 3+od, 4, 4+od all with syncho changes.