I would expect that using the same certificate authorities for servers is probably not useful for client authentication, although maybe it might be if the only thing you care about is the domain name (although, it shouldn't be; anyways, many clients might not even have a domain name).
But, if you really need to use certificates from the CAs anyways, you might ignore some of the fields of the certificate.
> you might ignore some of the fields of the certificate
A lot of software really doesn't like ignoring the constraints. You can make it work, but there's a good chance it'll require messing with the validation logic of your TLS library, or worse, having to write your own validation code.
I had partially written a program (in C) to parse X.509 certificates, but the part that is missing is the cryptographic stuff (to validate signatures, and also should be able to extract the public key for use by a separate TLS implementation). (I intended also to make it to be able to make X.509 certificates; the cryptographic stuff will be needed for that too, to make private and public key pairs and signatures. A separate library for cryptographic functions should probably be used for this purpose, if I would have a suitable one. A separate library should also be used for TLS (OpenSSL is rather confusing, and I want to use my own handling of the certificates but OpenSSL makes it too confusing to do that).)
But, if you really need to use certificates from the CAs anyways, you might ignore some of the fields of the certificate.