Some applications, such as surgical interventions, require that potential soft robots have the capability to alter their shape and enhance their force output on demand. This paper presents an antagonistic stiffening mechanism combining pneumatic actuation with tendon locking to achieve configuration- and stiffness control. Elongation of a soft pneumatic section, resulting from air actuation, is opposed by constraining the length of integrated tendons. These tendons can be locked in length by pneumatically activated levers at the base of each segment. Hence, tendon locking will not affect the configuration of other segments of a multi-segment manipulator. Our concept achieves a stiffness increase of up to 201.7% and a larger, more uniform radial workspace compared to the widely used pneumatic actuation concept while maintaining the low technical effort required for actuation. We also demonstrate how our actuation concept enables independent control of stiffness levels for individual segments of a multi-segment manipulator and their MR compatibility.