Extend X/Y A2S not working as separate X/Y Spline.
But, with RC5 you could get separation using Marker. Only one/first marker on start position will be used to get User_ Data, but not to affect geometry.
So, you could get a numeric value from the User_Data field (eg. percentage separator) for each rectangular subspline. However, this will also probably require some nesting and a different approach than the usual way.
Another way could be adding the numeric Parameter and using it as a value for Conditionals Nodes you already have on 50% to separate Left/Right.
Of course, some more Composing Operators should be introduced at that point to add the mullion segments.
From my experience, I would go with Marker+Nesting. You will get more freedom and a more open Style for future upgrades. Also, I would try to make Macros to create basic Window Frames.
Another thought on the topic. With RC you can make a very complex and fantastic generator. I`ve done numerous versions of Windows Generators based on clients' specific needs with a lot of parameters and features. The only thing to keep in mind is the scale of your project. A nested/complex generator could slow down on a very large-scale project.