I've been working as part of my service company for a client (ClientA) for a year now on building a platform. 

A few weeks ago another client (clientB) asked me for a quote for building a similar product. I said to clientB that we have been working on a similar product for a client and that we have a mature platform that they may use without all the hassles of doing it from scratch. So I sugested that they partner with my other client to use his platform instead.

Unfortunately, the negotiation didn't went well because of many factors:

1. ClientA don't want to have a partnership with ClientB but instead want to keep owing the platform and provide it to clientB as a SAAS service: which I can understand because of the investment already made.
2. The platform doesn't have all the features that ClientB wants, so they need to invest money in developing those. However they are reluctant to invest such money in a platform they don't own. They are planning to sell it to thousands of their own clients, so basing their whole business on a product they don't have control over is not a good move for them.

I've done my best to make them agree, but seems like they can't because of each one wanting to have control on the platform.

Now clientB wants, as he already wanted at first, my company to build a similar platform and wants my company to partner in this. We'll share any ROI with them.

It is a tough decision for me to take because I understand that we can't as a company keep working on those two platforms at the same time.

The first version of the platform was obtained from a freelance website under the "work for hire" clause, so the code belongs to clientA. So we cannot reuse any part of that work.

As a company manager, I'm in a dilemma because I suspect that clientA will take that move as an unfair one.

What are your thoughts on that matter?

Edit: There is no signed NDA and anyway would not have signed an NDA with a non competing clause.