There are various responses you could give. I think you should focus more on why each card is justified than on why two cards at once are needed.

Recently, I have explained to Chase that I needed a separate card for business expenses (this is most useful when you are applying for both a business and personal card).

You may also explain how your travel patterns would make the specific card particularly useful. For instance, if you live in Denver you would primarily fly United. Your company may use Hyatt as a preferred hotel partner, particularly in a city you regularly visit. You may be saving for a family trip to desination X, which is served by airline Y from your home/the nearest hub.

Stay away from the topic of sign-up bonus but focus on the benefits the card provides that will give you an incentive to put much of your spending on that card.