One way that springs to mind is to phase-lock them to the same reference with an at least type II PLL, so that they are in-phase, and then feed the one that should be lagging by 90 degrees to a quadrature hybrid. From that you can get the 90 deg lag.
I am sure plenty other ways exist, like rat-race couplers etc. to give you the required 90 deg lag.
There exists the issue of bandwidth ofcourse, as these networks will stay true to the 90 deg lag over roughly a 10% bandwidth.
If the signals are monochromatic (fixed frequency) then a simple λ/4 line will also do.