I'm wondering if it makes sense to find two cofounders, a business-oriented cofounder, and an engineering/product oriented one, or whether I should just go with someone who is a blend of the two? It seems to me to prevent infighting and differences, you should definitely try to minimize the number of cofounders, but I worry that someone with a mixed background might not be as effective a business leader or engineer.
You're correct, the fewer co-founders the better in terms of decision making complexity. The skills of a co-founder should be somewhat irrelevant. Remember that a co-founder is someone who shares an IDENTICAL vision for your company and product. Vision is the ideal experience of your product/company for customers, both external an internal. If you're lucky, a co-founder will have some complimentary skills. If they don't you're still lucky to find someone who shares your vision, and together you can find others to fill the skill gaps.
You should only seek a co-founder by the definition above. Trying to blend two or three things as requirements makes it even less likely you will find such a person. If a co-founder was a carbon copy of you, it would still be a beneficial relationship. With each co-founder you add, the complexity of decision making is exponential, not additive, and if you think finding one person who shares your vision was hard, finding more than one is even harder.
Engineering and product are not the same thing. One is a technical role, the other a marketing role. Business? That's too broad. There are six fundamental business skills, not the only ones, but the fundamental ones common to every enterprise. They are marketing, sales, organization, people, efficiency, and leadership. No one person will ever master more than two. You will always need support from employees, advisors, service providers, etc. to fill in the gaps. Do not assume that a person who joins pre-revenue is a co-founder. Being first doesn't confer any special rank. Sometimes what you need are employees, not co-founders. Being able to pay them is a completely separate issue from what role or relationship they should have.
Go for some one with mixed back ground. You will get the desired value out of him/her.