Getting "cannot set using a multi-index selection indexer with a different length than the value" error while using np.where function

I am trying to append two data frames row wise iteratively. After that I am trying fill 0 values in one column with the values in other columns and vice versa. I am using np.where function to fill the 0 values. When I am doing it separately it is giving correct result but when I am using it in a loop it is throwing "cannot set using a multi-index selection indexer with a different length than the value" error. My code looks like below.

def myfunc(dd1,dd2,dfc):
n=dd1.shape[0]
for i in range(n):
    dfc2=dd1.iloc[i:i+1].append(dd2.iloc[i:i+1])
    dfc=dfc.append(dfc2)
m=dfc.shape[0]
for j in range(m):
    dfc.iloc[j:j+1,2:3]=np.where(dfc.iloc[j:j+1,2:3]==0,dfc.iloc[j+1:j+2,3:4],dfc.iloc[j:j+1,2:3])
    dfc.iloc[j+1:j+2,3:4]=np.where(dfc.iloc[j+1:j+2,3:4]==0,dfc.iloc[j:j+1,2:3],dfc.iloc[j+1:j+2,3:4])
return dfc

Where dd1 and dd2 are my dataframes, I am appending rows in them iteratively to a empty dataframe dfc. Here I am using row and column indices to fill the values. Any help on this will be appreciated.

dfc.iloc[:,2:3] = np.where(dfc.iloc[:,2:3]==0,dfc.iloc[:,3:4].shift(-1),dfc.iloc[:,2:3])
dfc.iloc[:,3:4] = np.where(dfc.iloc[:,3:4]==0,dfc.iloc[:,2:3],dfc.iloc[:,3:4].shift(-1))