I think it might be cute to make 1 invite for them both. Maybe on the cover put, ex. YOU ARE INVITED TO 1 BIG PARTY FOR 2 SPECIAL BOY. Then when you open the card on one side have the theme for the 5 yr olds party, and the other side have the theme for the 1 year old party.
I have been making card for my kids for the last year, and have actually started helping my friends with theirs. Email me if you would like some help. ____@____.com