The triggered abilities from Distended Mindbender and Deadbridge Shaman will go on the stack at the same time. Since you controlled their sources as they triggered (C.R. 603.3a, 112.8), you can order the abilities as you want (C.R. 603.3b). (In each case, you choose an opponent to target as you put them on the stack [C.R. 603.3d, 601.2c].) If you put Distended Mindbender's ability on top, you can do as you want.
In general, triggered abilities that trigger as a cost of a spell is paid (such as sacrificing Deadbridge Shaman) will go on the stack at the same time as triggered abilities that trigger when that spell is cast (such as Distended Mindbender's ability), even though both kinds of triggered abilities trigger at different moments. This is because players don't get priority here until the player finishes casting the spell (C.R. 601.2h-i), and triggered abilities go on the stack only at moments when players would get priority (C.R. 116.2a, 116.5). All of this applies to activating abilities as it does to casting spells (C.R. 602.2, especially 602.2b).
What I want to happen is that I get to choose and discard 2 before the opponent chooses their discard.
Thanks in advance
In general, triggered abilities that trigger as a cost of a spell is paid (such as sacrificing Deadbridge Shaman) will go on the stack at the same time as triggered abilities that trigger when that spell is cast (such as Distended Mindbender's ability), even though both kinds of triggered abilities trigger at different moments. This is because players don't get priority here until the player finishes casting the spell (C.R. 601.2h-i), and triggered abilities go on the stack only at moments when players would get priority (C.R. 116.2a, 116.5). All of this applies to activating abilities as it does to casting spells (C.R. 602.2, especially 602.2b).
EDIT (Jul. 24, 2017): Minor edit.