We have always done "combo" parties for (extended) family birthdays so that cousins, and grandparents could all be together. However, in my immediate family, our closest were 2 weeks apart so we had individual celebrations (dinner, cake, presents, etc.) for one person.
For a one year old, a party is really an excuse for you to get together with your friends or family to celebrate your daughter. If these are all people who would also celebrate your husband's birthday, go ahead and make it a combined party. However you decorate the cake on your daughter's end will be for you, the people there, and for pictures - your daughter is not going to remember one thing about this party. If the party is going to be mainly focused on her, decorate the whole cake that way, and then make an announcement that you have another birthday person in the house, and bring out some other dessert with a candle for your husband. If you are really celebrating both, a combo cake will be cute. You could either keep the 2 halves very separate with baby girl stuff, and manly man stuff, or you could do some kind of father-daughter theme. I would say how you decorate the cake would depend on the guests you plan on inviting and the focus of the party.