How to Smoothly merge two PerlinNoise

Hello there,
I'm working on a minecraft style game just for hobby and study reasons.
The thing is, i've been trying to achieve this as best as i could but the result isn't looking anygood.
The idea is to merge let's say a Mountain biome with a flatland biome

Right now the map chunks(Montain biome) are generated around the player, as he walks, once the camera reaches 100 meters of distance from the center it will start drawning the flatland biome.

this is my Mountain PerlinNoise setup

Code (CSharp):

1.  
2. if (bioma == Bioma.Mountain)
3.         {
4.             freq = 300;
5.             amp = 50;
6.  
7.             float o1 = (Mathf.PerlinNoise((offSet.x + x) / freq, (offSet.z + z) / freq) * amp);
8.  
9.             freq = 100;
10.             amp = 50;
11.             float o2 = (Mathf.PerlinNoise((offSet.x + x) / freq, (offSet.z + z) / freq) * amp);
12.  
13.             freq = 30;
14.             amp = 20;
15.             float o3 = (Mathf.PerlinNoise((offSet.x + x) / freq, (offSet.z + z) / freq) * amp);
16.  
17.             maxY = Mathf.RoundToInt(o1 - o2 + o3);
18.         }
19.  

And my Flatland:

Code (CSharp):

1.  
2. if (bioma == Bioma.Flatland){
3.             freq = 100;
4.             amp = -20;
5.             float o1 = (Mathf.PerlinNoise((offSet.x + x) / freq, (offSet.z + z) / freq) * amp);
6.  
7.             freq = 35;
8.             amp = 5;
9.             float o2 = (Mathf.PerlinNoise((offSet.x + x) / freq, (offSet.z + z) / freq) * amp);
10. maxY = Mathf.RoundToInt(o1 + o2);
11. }

How it looks without mergering

 

In order to get the the biomes merged I've tried using both values, reducing one as the other grows like:

(MountainValue *0.1f) + (FlatlandValue * 0.9f)
(MountainValue *0.2f) + (FlatlandValue * 0.8f)
(MountainValue *0.3f) + (FlatlandValue * 0.7f)
(MountainValue *0.4f) + (FlatlandValue * 0.6f)
...
(MountainValue *0.9f) + (FlatlandValue * 0.1f)

it did almost work, but the result was very weird, so i'd like to hear some ideas thx
How it looks without mergering

With my current mergering:

 

Well i did it with Mathf.Lerp, not perfect but it worked

 

You may find this useful, not to mention the many other amazing things presented on the same site: http://www.iquilezles.org/www/articles/smin/smin.htm

 

Very interesting, definitely something i have to checkout, we never know what ideas might come up in the future hahaha