It really would depend on each startup, what its scope is, how inter-related (or not) each startup is to the other.
You are right that there is a definite risk of under-founding one or both startups.
Typically, there are too many things to be accomplished in a startup, which is one of the main reasons you are looking for a cofounder anyway, to help with complementary skills. In the case of co-cofounding, as you put it, you are dividing your time and thought processes between two different and competing ideas.
In your limited hours in a day, which one would you be (or should be!) most focused on? Your own idea! And the same holds true for your co-cofounder.
A better structure would be to help each other out (with reduced expectations of how much time and effort each would be putting into the other endeavor). If one project has more traction than the other, one of the co-cofounders may be tempted to drop their project that is not gaining as much traction, and focus all their efforts on the other one.
In that situation, you would become cofounders in the true sense.
It doesn't make sense to split your time completely between two projects - both are going to suffer.
Each project is a like a person that wants to share the same cake - your time. If you are spreading it out over multiple projects to minimize risk, you just end up giving each project a smaller piece of the pie - instead of one project feeling satisfied and full, each project is going to feel hungry - I don't think you are minimizing risk in that case.
Better to validate your idea with full focus and *then* move on to a next project *if* the business potential or customer need of the project does not pan out.
Just my 0.02.