If you are planning on hosting a joint birthday party for two kids who are around the same age, but different genders, this is completely doable. A decision you will have to make while planning includes if you want to plan two different mini-themes(one for the boy and one for the girl) or one general theme that can be used for both boys and girls.
If you host two mini-themes under one joint party umbrella, then it is appropriate to run different crafts, games, and activities for each of the kids, and group them by boys and girls. This is an especially good idea when the kids are in primary school and have a very good sense of what they want for their party themes.
Now, if these kids are under the age of three and they are a boy and a girl, it may be better to choose a joint theme and celebrate it as one thematic party for the kids. For one thing, the birthday kids will not know the difference, second, it will save time and money in planning, and there are many themes that can be chosen that will work well for boys and girls at this age. Some themes include: initial, alphabet, colors, barnyard animals, etc. The colors for decorations will be neutral for both boys and girls and simple activities can be done with the group that will be enjoyed by both boys and girls.
More than genders, it seems that age is the important factor when making this decision.