If two countries have identical production possibility frontiers, then is there still incentive for the 2 countries to trade?

Yes, because trade happens between individual businesses and people, not between countries per se, and there are more and more tradable good and services whose transaction costs are tiny compared to their total value, for example IT services, small electronics, virtual software and add-ons like skins in video games, R&D, new medicines, and much more.

If item X can be produced at the exact same price and quality in two different countries, and the cost of sourcing and shopping is low enough to make the total cost to the buyer almost identical, then other factors beyond quality and price will influence different buyers in different ways. For example, a health care company might advertise that they use only imported equipment in order to appear more prestigious and more reliable, even when the same quality of equipment could have been obtained at the same price locally.

A greater range of suppliers means more ways to differentiate products and this provide great value to customers with different priorities. Not only that, but the increased competition motivates suppliers in different regions to continually increase their value proposition but raising production efficiency and product quality.

So even where two countries have identical production possibility frontiers, or even identical current production profiles, allowing trade between companies and individuals in those two countries, with as little friction as possible, is still in the great interests of the consumers of those countries.