You are able to allocate in any way you choose, as long as you all agree. If you don't agree, then the default it the proportionate percentage based on people on the plan. In other words, if there are four people on the plan, 3 on the same tax return, and one not, the default if you cannot agree amongst yourselves is 75/25.
This article may be helpful: I'm on my parents' 1095-A form. What do I do on my return?
