split – Box two chunks of text side-by side
The widths of the boxes are set by the user.
The operation is reminiscent of the sort of thing the LaTeX package parallel does.
Maybe you are interested in the following packages as well.
- automagic: Automagic numbering in Plain TeX
- outerhbox: Collect horizontal material for contributing to a paragraph
- letter-plain: Letter macros for Plain TeX
- lettermac: Simple macros for writing letters (Plain TeX)