I know it can be hard finding people to fill all of the callings in a ward, but I want to stress to you the importance of two-deep leadership in the Scouting program.
First and foremost, this is a "Youth Protection" issue and a legal issue. It helps protect the Church, you and the leaders from litigation issues. More importantly, it helps protect the boys. I am sure you have seen news stories about lawsuits against Scout leaders, the BSA and the chartering organizations for untoward conduct. Many cases even involve LDS leaders.
There are other reasons, though, that two-deep leadership is essential. Running even a small den of three or four boys is difficult for one leader alone. Often there is a boy who needs special one-on-one attention for many activities, and an Assistant Den Leader can help with that. An Assistant Den Leader can also help make things run more efficiently. For example, he or she can check the boys' books and mark the records so that the Den Leader's attention doesn't need to be diverted away from the boys or the business at hand.
Consider also the "train your replacement" aspect that goes with any calling. Having two people working together ensures that if one needs to leave the calling, for whatever reason (which will happen eventually), there is still someone there who knows what's going on. This is why it's nice to even have positions like Assistant Cubmaster filled, as well as extra Committee Members.
If the pool of willing people in your ward is so thin that you absolutely cannot accomplish two-deep leadership for every den, it might be time to consider combining meetings with other wards in your building. I have seen these two and three pack committees work very effectively. Either that or parents can be encouraged (or even called) to take turns attending meetings until an Assistant Den Leader can be found.
Remember also the importance of having these leaders registered right away with the BSA and properly trained. Church Leadership has stressed the importance of these two things and has come out and said that no one should work with the boys until he or she is registered with the BSA.