Algorithm:

1. Define two variable max & sec_max
2. Initialize max to array[0] & sec_max to INT_MIN
3. Scan the entire array

a. For(int i=0;i<n;i++)
    if array[i]>max
    then update max to array[i]&sec_max to previous max value
    else if array[i] is greater than sec_max but less than max
        then update sec_max to array value
        do nothing to max
End for loop

1. If still sec_max has the value INT_MIN
   Only one largest value exists, print it
   Else
   Print max & sec_max

C implementation to find two largest elements in a one dimensional array

#include<stdio.h>
#include<stdlib.h>
#include<limits.h>

void findTwoMax(int* a,int n){
	int max=a[0]; //initialize max to first array element
	int sec_max=INT_MIN,temp=0; //initialize max to first array element

	for(int i=0;i<n;i++){ //scan the entire array
		//if a[i]> max then update max to array value & 
		//second max to previous max value 
		if(a[i]>max){
			sec_max=max;
			max=a[i];
		}
		//else if a[i] is greater than second max but less 
		//than max then update second max to array value
		else if (a[i] > sec_max && a[i] < max) 
			sec_max=a[i];
		//do nothing to max
	}
	//if second max is still at its initialized value 
	//then no second maximum exists at all
	if(sec_max==INT_MIN)
		printf("only one large element: %d ",max);
	else
		printf("First largest element: %d  & second largest element : %d",max,sec_max);
}

int main(){
	int n;
	printf("enter no of elements\n");
	scanf("%d",&n);
	
	//dynamic array created
	int *a=malloc(sizeof(int)*n);
	
	printf("enter the elements........\n");
	//taking input
	for(int i=0;i<n;i++)
		scanf("%d",&a[i]);
	
	//find two largest no
	findTwoMax(a,n);

	return 0;
}

Output

First run:
enter no of elements
4
enter the elements........
32 54 -6 43
First largest element: 54  & second largest element : 43

Second run:
enter no of elements
1
enter the elements........
4
only one large element: 4

Third run:
enter no of elements
4
enter the elements........
12 12 12 12
only one large element: 12