Single-Ended Signals

The best way to split single-ended signals into two copies is by using passive power splitters or dividers because they do not add noise or jitter to the measurement. Resistive dividers are a little simpler to use than reactive dividers because they do not have a low frequency cutoff and generally they have a flatter frequency response. Resistive dividers do attenuate the signal more than reactive dividers (-6 dB instead of -3 dB), but that rarely matters for noise-reduced oscilloscope measurements of clock signals.

Much less common is to split a single-ended signal into a differential signal using a passive balun transformer.

You can also use buffers or amplifiers to fan-out your Signal Under Test (SUT), provided you ensure the buffer or amplifier does not add appreciable jitter of its own to the measurement. Even the two differential outputs of a differential buffer can be used if the buffer does not have significant common-mode noise.