Those culverts seem full enough in the pics that I would stick with two and not go down to only one. Keeping two culverts should cut down on the water level at the culverts, which in turn should cut down on the beating that the banks take, and cause less erosion.
The creek seems to have a bit of an angle heading into the culvert on the upstream side, which means that the force of the water's flow will bounce off the coincident bank somewhat (first pic, across the creek on the right side of the culvert). Concrete would be good on the right side of the culverts as it would better absorb the force of the water and reduce (if not eliminate) erosion.
If it were me and I was mindful of a budget, I would:
- buy two collars and two more sections of culvert (or one section and half it);
- dig up the current culvert;
- attach additional length of culvert to each culvert (using the collars);
- re-center the culvert to the road;
- re-build the road a bit wider with a gentler grade;
- pour concrete on the upstream side to help reduce erosion; and
- pour concrete on the downstream side as funds permit.
It's like my buddy John told me when we were discussing how much topsoil to replace in my yard, "People who can't afford to do it right the first time, can always afford to do it right the second time." Loosely translated, "It's cheaper to do it right the first time."
HTH