I looked at the Tradelog website. You need to figure out if it is worth it to have the pro account. The clients I have with multiple accounts tend to have stocks in one and bonds in another, for instance. So I wouldn’t go for it myself. However, if you have a handful of clients for whom it is essential, I imagine you would want to price the fee at a level that makes having their own account uneconomic. I’m on my cellphone so I don’t have easy access to their website, but I doubt you’d want any fewer than five clients on this.
As I recall, all plans come with one “free” tax ID. If I were you, I’d reserve that for myself and charge a portion of full price to the clients. Precisely how you do that will depend on how you bill. I charge by the hour but if I needed specialist software for a few clients, I’d be inclined to charge a small annual fee for access.