In 80% of use cases, truncating a field to a specified number of characters makes sense for teaser display modes. For those few use-cases where you actually want different content, what you really should be using is a different field.
Having these two concepts contained in a single field is confusing for users, and will become even more confusing when we introduce a Rich-text editor.
We may need to include some kind of an upgrade path for sites that were using the splitter before, but I think that could be done reasonably easily.
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A simple way — use Field Group to create a container for fields https://backdropcms.org/project/field_group and CSS Flex оr Grid to alignment.
Layout also for /node/add/ forms?
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Layout also for /node/add/ forms?