Splitting Retirement Contributions When Married

[Gilly]

I have access to 457(b)-$100 employer match
Spouse has access to 401(k)-5% employer match
We max our IRAs. However, we are unable to contribute up to the combined max of our above plans. If we were able to contribute to the maximum clearly this wouldn't even be a question.
Obviously we will contribute enough to meet the matches.
Our funds options are similar.
I’m looking at the fee structure of account management, but both appear to be relatively reasonable and close enough that I would need to compare side by side, and the advantage of which has the better fees may change with amount invested.
 What other factors would you consider when contributing to the individual accounts? Are there any advantages to a 457(b) vs a 401(k) that would make one preferable?

For convenience sake, say you can contribute 20k total between the two accounts. How would you determine what amount get contributed to each? Would you send 10k to each, split it by proportion of income, ie if income is 60/40 contribute 12k/8k, or some other option? Why? If one had much better fee structure of investment options would a 18k/2k split would be the most optimal assuming 2k was enough to get the 5% match? I'm not looking for judgments on how couples manage finances, just options and things to consider.

[MDM]

What other factors would you consider when contributing to the individual accounts? Are there any advantages to a 457(b) vs a 401(k) that would make one preferable?

You have covered the important factors.
With a government 457b (that usually includes K-12 school districts), one can access the money penalty-free (not necessarily tax-free) as soon as you one retires.  Not always so with a 401k.  See Advice for 457b distribution - Bogleheads.org for drawbacks of non-governmental 457s.

For convenience sake, say you can contribute 20k total between the two accounts. How would you determine what amount get contributed to each? Would you send 10k to each, split it by proportion of income, ie if income is 60/40 contribute 12k/8k, or some other option? Why?

$10K each, because "share and share alike" seems a good default.

If one had much better fee structure of investment options would a 18k/2k split would be the most optimal assuming 2k was enough to get the 5% match? I'm not looking for judgments on how couples manage finances, just options and things to consider.

Yes, financially.  Relationship-wise, it depends....