As you may be aware by looking at it in design view, a split form is in fact a modified single form.

For that reason, I don't believe it is possible to achieve what you want using VBA. There are many limitations in what you can do due to the way that built-in split forms are designed. These include several different display issues, runtime issues and code issues. See Split Form Issues for details

I would recommend you replace it with your own form that has similar functionality but allows you more control over the layout. The above article also includes links to several alternatives.