I'm not sure what you're asking but, first, there is no RCA optical output or cable that I'm aware of. Optical cables on today's devices have either a glass or plastic cable with a Toslink connector or mini-Toslink connector at each end. A digital cable with RCA connectors at each end has a copper wire conductor inside the cable.
If what you're trying to do is connect the Oppo to both the Audio Research and MHDT DACs you could connect the Toslink output from the Oppo to one of the DACs and the RCA output from the Oppo to the other and on select which output is active from the Oppo.
If what you're trying to do is connect the Oppo to both the Audio Research and MHDT DACs you could connect the Toslink output from the Oppo to one of the DACs and the RCA output from the Oppo to the other and on select which output is active from the Oppo.