You can plant them as a unit IF you only plant one pot per hill or very large container. If you're doing a line along a trellis, then you need to cut off all but one to a pot.

Or, rather than sacrifice some seedlings, you can pull the clump apart and plant each one separately. A few broken roots won’t hurt them.

Ideally you should have one plant, but many people do plant pairs and still get perfectly fine food. Maybe, with pairs, you might get more cukes/ zuccz initially during the first production, but the plants may die faster. But cucumbers and zucchini plants usually decline badly or die before the end of summer anyway. Really, your choice. You may not need a bunch at one time maturing, or maybe you do.

Break off or cut off the stems of the unwanted seedlings. At this point you want at least one strong seedling, not stressed out multiple plants in the summer heat.

Thanks so much to kate, floral_uk and rbrt!