The right oscilloscope for small phase shift
If you want to see 5ns timing difference between your two 100Hz 2Vpp sinewaves from a single cycle observation, you have taken a hopeless task. At zero crossing the voltage difference of those sinewaves is about 3uV. You really cannot expect to see reliably that low voltage differences in ordinary oscilloscopes. The signals aren't sharp pulses but slooooow ramps.

24 bit voltage resolution (1V full scale) would in theory allow about 2% accuracy for that 3uV if the timing resolution is in accordance and the noise is filtered to low enough with band limiting and long averaging. The needed timing resolution is about 5ns/50 = 0,1ns. That means 10GHz sampling rate.

Forget it. Or specify some lower needed accuracy "how much error is allowed to the 5ns timing difference".  Until this you have specified nothing. 18bit resolution is given, but not said what range must be quantized to 18 bits.

ADDENDUM:

Commentator @Neil_UK proposes to get smart instead using brute force. So, in theory you for example could generate sharp pulses from the zero crossings, extract a high harmonic and get more easily measurable phase difference from that 5ns time shift. That's possible in analog domain. But to get it properly extracted from the noise needs a signal processing mathematician to help. That is well beyond my zone of comfort.