Dear Balaji
Am not sure whether these both can be mingled. But because of the tim and work constraints it will be better if done one two separate days. For ganapathy homam itself we need rava kozhukattai, adhirasam and all other things. For sumangali prarthanai, if ladies start atleast the function by 9 then only we gents can eat atleast by 12:30 or 1. Also, if they start cooking by 5-6 then only they can finish it by atleast 9. This and all I practically saw when my mom and chitthis together performing sumangali prarthanai. (They have to prepare that much items with complete madi na). If the ladies of your house is OK with the proposal of combining both, then I think there wont be any issues. Because, ganapthy homam is performed usually in early morning and this prarthana is performed near noon only.
Pranams