Freelancers working together - invoicing

We are two self-employed freelancers in the service industry. We wish to work together as on-going exchange of skills on varied projects. Freelancer A would be dealing with the client and manage the projects, whilst Freelancer B would only be supplementing the work of Freelancer A.

The main objective is that the client will only receive one project invoice for all services, rather than both individually invoicing the client. The reasons for this are a) low likelihood of being commissioned individually if services are offered separately b) do not wish to essentially be competing against each other, as separate suppliers

Assuming Freelancer A sends an invoice for 1000 Pounds to the client, and Freelancer B invoices Freelancer A for 500 Pounds for services provided for this project, would this mean that Freelancer A is paying Income Tax and NI on those 1000 Pounds, as well as Freelancer B paying Income Tax and NI on the 500 Pounds? Or could Freelancer A offset this as cost or expenses and only be taxable on the 500 Pound share?

We would be mostly working together, as well as have some individual projects. Aside of Income Tax and NI issues, would we run into a disguised employee situation if this was a long running arrangement?

We don't wish to yet set up a company before knowing this arrangement will yield more work for both of us. So in a nutshell: is it a legal set-up to work like this? Would we pay extra tax and what could be an alternative?