I think the developer team could sell tokens with ICO and IEO simultaneously. But I think it will be more effective to use the IEO method because of the current trend. ICO and IEO simultaneously only waste huge costs.
What is the point having both together, ICO is for those that has zero capital to start with, so they can as well go for ICO and then wait for as long as they wish till they establish some sort of success in their campaign, while IEO is basically for projects that already has start up capital, because only projects that has startup capital can pay such amount of money they pay for listing on an IEO platform, but the payment is actually worth it because they tend to meet up with their hardcap within the shortest period of time.
The only issue I have with IEO is that I just feel these exchanges are exploiting more than even in ICO, because I really dont see what they do to have been taking such amount of money to just list a project, it is the biggest scam I have ever seen in its cryptocurrency market that people are not looking into.