>>What is your basis for splitting the refund?<<
There are several formulas available, and it certainly isn't limited to refunds. it is actually a three-step process. First allocate the income and deductions. Then determine how much of the total tax liability to allocate to each spouse. Then allocate the withholding and estimated payments to see if one spouse already paid too much. You'll have to decide where to put various credits. Keep in mind that clients can agree to one of these splits regardless of how they actually file. They may want a lower overall tax, or they may want to avoid joint liability.
Don't try any of these without an informed waiver of conflict of interest, in writing. If you run into trouble, you might be able to get back on track by talking about the two bank accounts for direct deposit.
One of the best ways is 50/50. It is simple to do and easy to understand. I justify it on religious terms, community property, low cost, running out of time, postponing hard choices, lawyer's instructions, de minimus effect, or whatever fits the situation.
Form 8958 is an official IRS calculation, so it has the feeling of objectivity. It is designed for community property but can be used as a worksheet in any state. It recognizes that some things in a marriage are joint while some things remain separate. The allocation can be made according to law or agreement. The brief instructions are helpful. Formal isn't necessarily better, so variations are to use your own Excel worksheet or just pencil and paper. Usually I will do a quick pencil draft and quote a fee for a more detailed analysis.
Here's a model I've used once or twice but don't like. If one party refuses to share I will draft either an S or an MFS return using just those items. The idea is, "You have to file anyway, so how much extra did my information add?" Or maybe, "I won't pay more than if I were single." This can be very misleading sometimes. I mark the printout draft and planning-only, because it does not follow tax law and can't be used for filing. Nevertheless I charge full price for it. .
A variation of that is what I call tax effect. Some items are inherently unequal, such as capital gains with a lower rate, claiming kids for EIC and Head of Household, and high income subject to phaseout or AMT. You can use a special allocation to compensate for this. It sounds harder than it is, and I generally just do a partial for the item in question. I only do it on instructions from one or both attorneys, and I don't bother billing because they won't pay anyway.