I saw the same sub-problem in LeetCode, and there exists a O(n) neat greedy solution:

for _ in range(int(input())):    a = input() + '['    b = input() + '['    output = ""    for _ in range(len(a) + len(b) - 2):        if a < b:            output += a[0]            a = a[1:]        else:             output += b[0]            b = b[1:]          print(output)

Please note: '[' is the first char after 'Z'.