Again--the W-4 you fill out for your employer does not dictate how you file your tax return. What it affects if how much tax you have withheld from your paycheck. You could both use the W-4 calculator to help you determine how much you want to have your employers withhold. Since you are entertaining the idea of filing separately, you should both try out the tax caster tool for married filing jointly and married filing separate to get an idea of the effect those filing statuses have on your next tax return(s). The tax caster tool is currently geared for 2016, and some tax laws may change for 2017 tax returns; but it should give you a pretty aged idea of where you would stand.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
