I can do split screen between apple maps and apple music when using an iphone.
I can do split screen between google maps and youtube music when using an android.

However, when I'm using the Lexus map and Satellite radio, I cannot get it to split screen. I have to keep switching and can't have both side by side.
This is so annoying. Anyone know if I have to press a sequence of buttons or something? I have the 12.3" screen.

 

The "old" Lexus multimedia system in 2019 through 2022 model years ES were/are equipped with does have such a split screen capability; however, the "new" Lexus Interface system in 2023 and later model years ESs does not have split screen capability when using the native Lexus interface.

The reason you're getting split screen functionality with Apple CarPlay and Android Auto is that the phone hosted software for those two systems is what composes the display items and layout of the video frames that are displayed on the ES's center LCD display. Both of those systems' software has split screen capability, so that's why you can have split screens with them and not with the "new" Lexus Interface system.

 

You can change the Instrument Panel layout to display the media information next to the speedometer
but it is smaller than the split screen that was available next to the Map previously.

 

Doing away with the split screen is another of Lexus's bright ideas if you have a 2023 up. It might be brought back with the 2026.

 

So, if OP has a 2021, which I thought he previously mentioned, then he should have split screen capability?

 

Yes, with the pre-2023 cars, the "hard keys" that you can press with your gloves on:
Map, Menu, Radio and Media, control the left side of the screen. The "soft keys" that
you control with the remote controller/touch pad/touch screen, controls the 1/3 right side.

Within the right-side displays you can switch displays such as Map to large Compass,
MPG to Hybrid Energy Flow. The right-side HVAC panel is the quickest way to disengage
the A/C compressor when driving with the sunroof/windows open. Just tap the Green A/C!