If, as a company, we decide to join as a Retail Provider and then later on we decide to join as an Access Provider, do we have to sign up for the two things separately or we can we sign up for one and then add the other one to the same membership at a later date?

If, as a company, we decide to join as a Retail Provider and then later on we decide to join as an Access Provider, do we have to sign up for the two things separately or we can we sign up for one and then add the other one to the same membership at a later date?

They are two separate subscriptions. Access Providers require the ‘Wholesale’ service and Retail Providers need the ‘Switching’. If a company decides to join as a Retailer and also wants to become a Wholesaler, it will require a second subscription.
The two functions are intentionally set up this way because there's an Ofcom requirement to separate the network from the retail side of things and therefore CWP/ CSP have created that separation in the platform. 
In addition, one team in a company may deal with Wholesale and another with Retail. These two teams don't meet on the platform.