You cannot split irregular frame in the same way as a regular frame. Doing so, the irregular frame would be replaced with a rectangular split. The trick is to use a tall, narrow frame as a wrapper which cuts in two parts the irregular frame.

On the figure bellow, text wrap is activated on the blue frame, which is also in the foreground. The wrap causes text in the red frame to flow in two columns.

If you need to split in more than two columns, just use more split-frames.