No, having filed separately last year doesn't affect what you enter this year. If you're filing jointly now, you'll simply enter the total of your expenses, like mortgage interest, instead of splitting it,because you'll likely only have one 1098 reporting interest for the mortgage.
When you file jointly, most items are entered by the form, meaning you may have only 1 form for 1099-DIV because you have a joint account. That's fine, because you'll check it's owned by both and enter the amount. If you have two separate 1099-DIV forms because you each have your own account, you'll just check the spouse to whom the first one belongs and enter the amount. Then you'll choose to Add Another and select the spouse who owns that one and again, enter the amount from that form.
The only real exception is for any state tax refund you received, where you do just add your two forms together and enter one amount.
