Welcome to planDisney, Patricia!
What could be happening is the remaining tables that are currently available at both
Oga's Cantina and
Sci-Fi Dine-In Theater may be larger tables that seat more than two guests. I have run into the same issues when I am booking a larger party. There may be two tables of four available, but if I have a party of eight, the system is looking for one table that can accommodate eight people and won't let me book two tables and split my party. I have been able to call a reservation cast member to book my two tables in those cases. They may be able to see if they can locate a table that would work for the two of you, Patricia. But, there is no guarantee.
If you need a back-up plan for dinner at
Disney's Hollywood Studios, I would suggest trying
50's Prime Time Café. This restaurant's decor and theme will take you back in time as your extended family serves you at mom's table! Be sure to try Aunt Liz's golden fried chicken and wash it down with a delicious Peanut Butter & Jelly Milk Shake!
Have a magical day, Patricia!
-Rachel