Sweetcorn

You can grow two if they're the same type.  Discovering the types of each seed can be problematic, the seed merchants don't always specify.

This is the main issue, I think. All sweetcorn varieties should be clearly labelled as su/se/sh2/syn, and all seed sellers should have a little infographic showing which types can be grown next to each other. 

Then you have a variety like Swift which is variously described by seed sellers as 'extra tender', 'tendersweet' or 'extra tendersweet', which is overly confusing since it's still sh2, just more tender aka 'augmented sh2'.