I think you encounter the problem mainly because your younger chiild is still too young. For me, my two girls are 7 and 5.5 years old. So I usually involved both in activities like reading, writing.
Try to give each child his/her own book or activity sheet so that he/she doesn't feel left out. E.g. if you're doing art & craft, make sure each child has his/her own set of materials. They need not be the same, can be different level of difficulty depending on their age, but of similar nature. You can also let each child have his/her set of stationery and write their names on their books so that they feel they're involved.
dont't worry, things will get better as they get older.