2 blade fixed broad heads have no bearing on jumping the string, the arrow speed determines how fast it gets there, not what kind of point/broadhead is on the end of it, so jumping the string with a fast arrow with a fixed OR mechanical 2 blade will be the same.
"A" reason for a fixed broad head as that it is much more likely to completely penetrate an animal than a mechanical one that might bend, break, fail etc and not cut much or penetrate much. also, fixed broadheads will not fail to "open", they are generally narrower but because of that, penetrate better, in any animal.
will a failed mechanical actually cut more than a fixed? will a working mechanical actually penetrate better (therefore cut more) than a fixed?
a buddy of mine shot a bull moose last season with a mechanical (4 times) and said he would never use them again on a moose. i suspect as the size of the animal gets larger, then the fixed blades offer more advantage.
just some thoughts (i have by the way killed multiple moose, caribou, black bear and black tailed deer with a bow)