Wow. Really? Ridiculous.

No, there are not other options besides buying at least one seat. I have never seen a situation where more than one unticketed infant is allowed per adult. That would be crazy.

Also, I'd consider long and hard how you are going to possibly help the second seated child to cut up her meal and eat, change diapers, etc. while you've got the other child on your lap. This really doesn't strike me as having any easy answers.