LIS on two arrays

Question

I feel lost on how to approach this question,

Given two integer array of size 𝑛 , 𝑚 , I want to merge these two arrays into one such that order of element in each array doesn't change and size of their Longest Increasing Subsequence become maximum. Once we choose an element of A or B, we cannot choose an earlier element of that sequence

My goal is to find maximum possible length of longest increasing subsequence. This is what I have so far:

def sequences(a, b, start_index=0, min_val=None):
    limits = a[start_index], b[start_index]
    lower = min(limits)
    higher = max(limits)
    if min_val is not None and min_val > lower:
        lower = min_val 
    options = range(lower, higher + 1)
    is_last = start_index == len(a) - 1
    for val in options:
        if is_last:
            yield [val]
        else:
            for seq in sequences(a, b, start_index+1, min_val=val+1):
                yield [val, *seq]

for seq in sequences([1,3,1,6], [6,5,4,4]):
    print(seq)

However, this results in: [1, 3, 4, 5], [1, 3, 4, 6], [2, 3, 4, 5], [2, 3, 4, 6]. The expected output should be:

array1: [1,3,1,6]        
array2: [6,5,4,4]     
We take 1(from array1), 3(from array1), 4(from array2), 6(from array1)

Giving us LIS: [1,3,4,6]. 
We got this by not choosing an earlier element from a sequence once we are at a certain value.

How do I stop it from unwanted recursion?