...but the 32-bit CAN find the 64-bit JDK.
When I open Android Studio I receive
Error launching Android Studio No JVM installation found. Please install a 64-bit JDK. If you already have a JDK installed, define a JAVA_HOME variable...
I have the 64-bit JDK installed with both PATH and JAVA_HOME set (confirmed this in cmd prompt)
PATH
...; C:\Program Files (x86)\Java\jdk1.8.0_91\bin
JAVA_HOME
C:\Program Files (x86)\Java\jdk1.8.0_91
I can, however, launch Android Studio by going to its bin folder and running the 32-bit launcher (studio.exe instead of studio64.exe).
Why would the 32-bit Android Studio be able to find the 64-bit jdk, and not the 64 bit AS?
Android Studio 64-bit works on both 32-bit and 64-bit JDK. Depending on your JDK installation you select the launcher:
studio.exe is the 32-bit launcher.
studio64.exe is the 64-bit launcher.
From your JDK path it's clear you have installed the 32-bit JDK. However, multiple JDKs can co-exist in one system so you can also install the 64-bit JDK if you wish.