C语言文件内容交错排列

想问一下大家怎么实现两个TXT文件里的内容交错排列然后输入到第三个TXT文件里啊😣😣
举个例子：
1：asd 2：qwe 3：aqswde
asd qw aqswd

#include <stdio.h>
#include <string.h>

int main() {
    FILE *file1 = fopen("1.txt", "r");
    char a[1000];
    fgets(a, sizeof(a), file1);
    fclose(file1);

    FILE *file2 = fopen("2.txt", "r");
    char b[1000];
    fgets(b, sizeof(b), file2);
    fclose(file2);

    FILE *file3 = fopen("3.txt", "w");
    char c[2000];
    for (int i = 0; i <= len(a) ; i++)
    {
        c[i * 2] = a[i];
    }
    for (int i = 0; i <= len(b); i++)
    {
        c[i * 2 + 1] = b[i];
    }
    fputs(c, file3);
    fclose(file3);

    return 0;
}

#include  <stdio.h>
#include <string.h>

int main(void){
    

    char str1[2000];  
    char str2[2000];  
    char str3[4000]; 
    
    
    FILE * fp1 = fopen("TXT1.txt","r");
    if(fp1==NULL){
        printf("文件打开失败！\n");
        return -1;
    }
    
    FILE * fp2 = fopen("TXT2.txt","r");
    if(fp2==NULL){
        printf("文件打开失败！\n");
        return -1;
    }
    
    int i=0;
    char ch;
    while(feof(fp1)==0&&i<2000){  //读取第一个文件的每一个字符 
        
        int num=fscanf(fp1,"%c",&ch);
        if(feof(fp1)==0&&num!=0){
            str1[i]=ch;
            i++;
        }
                
    }
    if(str1[i-1]!='\n'){
        str1[i]='\n';
        i++;
    }
    str1[i]='\0';
//    for(int z=0;z<i;z++){
//        printf("str1[%d]=%c,%d\n",z,str1[z],str1[z]);
//    }
//    printf("str1=%s\n",str1);
    fclose(fp1);
    
    int j=0;

    while(feof(fp2)==0&&j<2000){  // 读取第二个文件的每一个字符 
        // https://www.runoob.com/cprogramming/c-function-fscanf.html
        int num=fscanf(fp2,"%c",&ch);
        if(feof(fp2)==0&&num!=0){
            str2[j]=ch;
            j++;
        }
            
    }
    if(str2[j-1]!='\n'){
        str2[j]='\n';
        j++;
    }
    str2[j]='\0';
    
//    for(int z=0;z<j;z++){
//        printf("str2[%d]=%c,%d\n",z,str2[z],str2[z]);
//    }
//    printf("str2=%s\n",str2);
    fclose(fp2);
    
    int k=0;
    int len=0;

    while(k<i||k<j){  // 将两个文件的字符交错存入第三个数组 
        
        if(k<i){  
            
            str3[len]=str1[k];
            len++;
        }
        
        if(k<j){  
            str3[len]=str2[k];
            len++;
        }
        
        k++;
    }
    str3[len]='\0';
//    for(int z=0;z<len;z++){
//        printf("str3[%d]=%c,%d\n",z,str3[z],str3[z]);
//    }
//    printf("str3=%s\n",str3);
    fclose(fp2);
    
    FILE * fp3 = fopen("TXT3.txt","w");
    if(fp3==NULL){
        printf("文件打开失败！\n");
        return -1;
    }
    
    k=0;
    int  warpNum=0;
    while(k<len){  // 将已经交错排列的字符串写入第三个文件 
        
        char ch = str3[k];
        
        if(ch=='\n'){
            warpNum++;
            if(warpNum%2==0&&warpNum!=0){
                fprintf(fp3,"%c",ch);
            }
        }else{
            fprintf(fp3,"%c",ch);
        }
        
        
        
        k++;
    }
    fclose(fp3);
    
    return 0;
    
}

c1=fgetc(f1);
c2=fgetc(f2);
fputc(c1,f3);
fputc(c2,f3);