A repeater can be done in many ways, a simple proposal:
A 3 element cross-yagi pointing at sky, -> LNA+20 dB -> bandpass filter-> 5 meter coax cable -> Yagi pointing at your receivers.
As GPS signals are real weak must a good low noise LNA be used. A full size RX antenna improves total snr. If needed, additional gain is possible to insert in the end of coax cable if isolation is good enough between the both RX/TX antenna radiation patterns. 5 meter coaxial cable adds distance between the both antennas, which also helps to reduce unwanted self-oscillating.
If the GPS receivers can receive both direct and indirect signal from same satellite, is it possible that that satellite becomes blocked or results in unstable readings.