What Chris said :)
Splitting the APK into RPA files wouldn't decrease the installation time - it would in fact increase it. Unless you're hoping I separate out the RPAs into individual Acts, in which case I can't do that either as there's too many variables that rely on content from other Acts.
Also, as Chris says, it would be a lot harder for the casual user to install (which is most users - I already get two or three messages a week from people confused about how to install the APK) and I try to keep things as simple as possible for everyone.
I'm working hard to keep the filesize under 2GB, and it's looking promising. The game has been in development for nearly four years and has between one and two more years to go until it's complete, and it's still at 1.3GB, so... fingers crossed!
So, in short: no RPA splits are planned, although it's on the table if it becomes a necessity :)