A family is larger than a table: what can change?
Three intact families of five cannot fit at two eight-seat tables. At least one table would have to hold two families, and 5 + 5 is 10, which is more than 8. Sixteen chairs for fifteen people do not fix that.
What you can change on purpose
Add a third table with a capacity the venue has confirmed.
Use a larger confirmed table capacity, if one exists. Do not invent a ninth seat on an eight-seat table.
Agree to split a family, and write that split into the party list. Do not leave the must-stay-together rule in place and hope.
Enough chairs is a different question
The seat-count checker will say two eight-seat tables have 16 seats for 15 people. That answer is about chairs. This page is about intact parties.
Related
Capacity proof, not a customer chart and not a live solver result.
About 10 minutes. Software, not a waitlist — your chart tonight.