Obed - don't EVER do that. I know it feels like it's safer as it is cutting the span in half, but the problem is what can go wrong. Here's a story of a friend of mine. He was loading a small tractor into the back of a truck or trailer way back when he was a teenager. His Dad was helping. These are pretty experienced people around heavy equipment as his Dad was in construction and actually a degreed engineer too.
Well the inadequate ramps they had were not up to the task, so his Dad did exactly what you did - mid span supports with some blocks or bricks. My friend drove the unit up the ramps and all was fine until he got past the mid span supports. Once the load was in the upper section, it deflected the ramps enough that the bottoms lifted off the ground. Then the forward motion of the tires created the opposite reaction in the ramps, and caused the ramps to slip backwards off the vehicle platform as the mid-blocks had no lateral strength (loose blocks) to stop it (whereas the ground is solid and can resist that back sliding) ...and everything came crashing down, and I believe the tractor rolled to one side and threw him. This is one of those situations where he could have easily been killed.
But it seems logical - so what went wrong? The flaw was that the mid supports are loose (relying on gravity only) and not attached to the ramps. If you built a mid support that was sturdy and fully secured to the ramp, then you could get away with this, but people do just what you did - stack some blocks under there and go.
You are better off making/buying sturdier ramps that are up to the task. That is a main reason I doubled up my 2x8's. If you wanted them even stronger, a thin piece of steel on the bottom would help enormously too. Or a 2x4 on edge to make a T, especially near the center section. There is nothing I can move up those ramps that is going to break them.
It's an easy mistake to make, but now you know...