Filing a tax return means sending it to the IRS. You can only file once. But you can prepare tax returns both ways, without actually filing them, in order to do the comparison. VolvoGirl gave you some suggestions for how to do that.
TurboTax Online is not a good vehicle for doing the comparison. Each TurboTax Online account can have only one return, so you would need three separate accounts to prepare a joint return and two separate returns.
You could also save yourself a lot of work by just filing jointly. Unless there is something very unusual about your tax situation, it's extremely rare for filing separately to work out better. See VolvoGirl's second post for some of the disadvantages of filing separately.
If you live in Ohio and both have income, in some cases you might save enough on your state taxes by filing separately that it would more than make up for the extra federal tax. The only way to know for sure is to prepare federal and state tax returns both ways. But this is only for Ohio, because of the unusual structure of the Ohio state tax.
