To save a multi quote fiasco, my thoughts on having to press gear up and gear down together to drop the post and it being a recipe for disaster –
Maybe if you have shift buttons on either side of the bars it could work, otherwise I can see it being either a compromise to the shifting or just dangerous to the ride.
If you have to push both buttons together, can you do it with one hand without losing concentration or having to think about it too much. If not, crash time – potentially just when you need to be concentrating.
If you can press them together easily enough, how much time differential does the software allow before it shifts gears. If it’s too long, one of the main aspects of electronic shifting – speed – will be compromised. If it’s too short, gear change instead of dropping the post is potentially crash time again. Or gear change and then post drop, not quite as bad but still annoying.
All in all it is probably something that can be managed in theory but, when used in a race or an event where things need to happen quickly and instinctively, I’d much rather have a separate button instead of interfering with an action that is normally an instinctive move.