assuming you got column size compatibility between arrays, you must create an array with x quantity of rows, x being the sum of array1's rows and array's 2 rows (I assume you know these sizes, since you had to declare the arrays sizes). After that, you create two nester for loops to go through both arrays one after another. This way:
int sizex = 4, sizey1 = 2; sizey2 = 3;
int array1[sizex][sizey1]={{1,2,3,4},{2,3,4,5}};
int array2[sizex][sizey2]={{3,4,5,6},{4,5,6,7},{5,6,7,8}};
int array3[sizex][sizey1+sizey2];
for (int x = 0 ; x < sizex ; x+=1)
{
for (int y = 0 ; y < sizey1 ; y+=1)
{
array3[x][y] = array1[x][y];
}
}
for (int x = 0 ; x < sizex ; x+=1)
{
for (int y = sizey1 ; y < sizey2 ; y+=1)
{
array3[x][y] = array1[x][y];
}
}
this way, array3 would be:
{1,2,3,4}
{2,3,4,5}
{3,4,5,6}
{4,5,6,7}
{5,6,7,8}