How to accomplish initial Splitter and SplitItem layout with enaml?

I'm using the enaml toolkit and would like to know how to initialize Splitter / SplitItem layouts.

Below is some very simple sample code. I'd like the window to start with the left SplitItem taking about 2/3 of the window width with the right SplitItem getting the other third. I've tried a variety of constraints in a variety of places but can't seem to hit on what I need to do.

Window starts like this:

I want it to start like this:

from enaml.widgets.api import (
    Window, Container, Splitter, SplitItem, Html
    )

enamldef Left(Container):
    Html:
        source = '<center><h1>Hello Left!</h1></center>'

enamldef Right(Container):
    Html:
        source = '<center><h1>Hello Right!</h1></center>'

enamldef Main(Window):
    initial_size = (800,400)
    Container:

        Splitter:

            SplitItem:
                Left:lt:
                    pass

            SplitItem:
                Right:rt:
                    pass

Use the stretch attribute on each SplitItem. The initial widths will be proportional to the fraction of total stretch values. So for the left one, use stretch = 3 and the right one stretch = 1. This will allocate 3/4 of the space to the left side and 1/4 of the space to the right side. The stretch values must be integers, not floating point values since this is what the underlying toolkits are expecting.