I've just begun cycling 'seriously' again after a break of oh, let say 20 years. I find, this time around, I'm much more interested in using (what remains of) my 'motor' more efficiently ie. consistent cadence enabled by appropriate gear choices. At times, this requires shifting both the front and rear derailures. Now the question. Is it advisable to shift both front and rear derailures simultaneously? I've 'discovered' this technique and it seems to work well. The shifts are smooth and nothing suggests it's any harder on the equipment than shifting one, then the other derailure. Still, I have nagging doubts that I'm promoting premature wear and tear of my transmission. For whatever it might matter, my bike is equipped with Shimano 105 levers and derailures. I'd appreciate suggestions/experiences on this subject. Thanks!

I too have done this, for years. Perhaps I just don't know any better. My road bike is 105-equipped, and has not suffered in the least from... tandem shifting? Syncro-shifting? Whatever.

I can tell you this: If your bike's drivetrain is not clean, properly adjusted, and well lubed, you will find yourself flinging the chain off of the chainrings before too long. This only happenned to me once, and it was my bad... I had ridden in the rain earlier in the day, and by late afternoon, we were somewhat less than a well-oiled machine.

I have not noticed any other wear or damage, and this bike has some 17K miles on it.

I do this also on my hybrid... it is equipped with older RX100 parts, and circa-1989 Rapidfire shifters. Works like a charm, up or down through the ratios.

Also... older Campagnolo gruppos really hate to be treated like that. They seem to prefer slow, deliberate shifts.

Shifting tip
 

No matter what system you use, decrease the pedal pressure when shifting, especially when doing the "simultanous" gig.
Although Shimano,(etc.), says it's ok to keep pressure on, shifting with "power on" dramatically increases the wear on your system and increases the chances of dropping a chain or getting something caught and bending parts.
Shifting with power reduced tends to make the shift quicker as well.