Exactly anyone is free to join as many project as he can handle freely. It is good that he is working for both projects and will have to learn and share from other developers from other projects. I do agree both projects are doing well at exchanges in open market and that is enough.
If investors are involved it can also be a little bit critical. If someone raises money and I pay him to bring his best knowledge to the project I funded, and that person then starts/supports another project with the knowledge I partially funded, not sure if I like that too much...
Yep, in some way managing multiple projects at once can really affect the development at a certain time. But if the person is very competent and reliable why not.