1) shift before the climb, when you're going faster and you can allow some slack
2) Determine how big the gap is. For example, on my 53/39 bikes the gap between big and small rings equals 2 cogs. On the 52/36 and 50/34 it's 3.
3) slacken up slightly on the pedals and shift the right number of cogs, THEN shift the front. Should take about 1 second all told.
I have tried shifting simultaneously and it works 95% of the time. The other 5% the chain drops. So I try not to shift simultaneously anymore. This can be exacerbated by the chain being in need of cleaning and relubing. I don't mean really dirty, just about the time you'd normally clean it.
The key is not having to shift under pressure on the pedals.
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