Multimodal biometrics
To overcome the error rates that exist in any biometric modality, it seems intuitive that using more than modality will produce better results. That intuition is not correct.

Using two modalities will produce a FAR that is far lower than the FAR of the more superior of the two modalities, if acceptance is based on both modalities having to be accepted. Therefore there will be more rejections, and the FRR will increase. The same is true if the criterion is that both modalities have to rejected, then the FAR is increased.

This is true for systems that are fully automated, but multimodal systems offer a significant improvement for environments that have additional support, and where the biometric system is used as a filter. For example, at biometric access gate also manned by security personnel, people that are accepted by both biometric modalities automatically gain access, people that fail both biometrics are refused entry, and the few who fail acceptance on either biometric have to be screened by the security personnel, possibly having to provide alternative proof of ID, before being allowed access.

More at:
A Multimodal Biometric System Using Fingerprint, Face, and Speech
A new application of Multimodal Biometrics in home and office security system
Combining Multiple Biometrics
Design Approaches for Multimodal Biometric System
Multimodal Biometrics: An Overview