To compare scenarios you would have to make two separate test returns. You can make additional online accounts with the same e-mail as long as they have different user names and passwords. Remember that when filing separately, both spouses must use the standard deduction or both spouses must itemize, even if there are not enough itemized deductions to go around.
Usually the reason your refund appears to drop is due to the assumptions built in to the W-4 withholding system. If your W-4 assumes that you are the only spouse who is working, you will not have enough tax taken out.
Remember that withholding is only an estimate of the tax you owe, your tax is calculated on your return only. A large refund just means that you had too much taken out of your paychecks and didn't have it to spend during the year. In most cases, separate filing will cause you to pay more tax because many deductions and credits are reduced or not allowed when filing separately.
