This is normal, if we look CH1 alone signal is "clear" thin trace and after CH2 is there CH1 signal come "fat".
Ch1 alone samplerate is 1GSa/s and CH1+2 samplerate is 500MSa/s
This is one form of aliasing. Its name is "corner wobbling"
But there in image can see also other effect (so careful to inspect so that do not mix different things) what come from trace fine positioning and then also some false triggers due to signal shape and not optimal trigger setting. But it is other case.
Whole talk about "cross talk" in this case is wrong. In your image CH2 do not show anything, least with my poor eyes. My eyes "best before" day have gone long time ago)
Cross talk mean that other channel signal is leaking to other channel. If you set high level fast signal to CH1 and then not connect anything to CH2 you can see CH1 signal also in CH2 when you select enough sensitive setting. Try 5Vpp 100MHz to Ch1 (1V/div) and nothing to CH2. Then select 10mV/div for CH2. Now you may see signal also there and this is cross talk. Typically attenuation is less with higher frequencies. 40dB cross talk attenuation (voltage) means 1:100.
About different kind of displayed aliasing...
Principle is also explained here (I can not find now this much clear and better example)
http://cp.literature.agilent.com/litweb/pdf/5989-5732EN.pdfIn this paper look Figure 4. (note that display mode there is dots)
Figure 4 shows an example of a
500-MHz bandwidth scope sampling
at just 1 GSa/s while operating
in a three- or four-channel mode.
Although the fundamental frequency
(clock rate) of the input signal is well
within Nyquist’s criteria, the signal’s
edges contain significant frequency
components well beyond the Nyquist
frequency (fN). When you view them
repetitively, the edges of this signal
appear to “wobble” with varying
degrees of pre-shoot, over-shoot, and
various edge speeds. This is evidence
of aliasing, and it clearly demonstrates
that a sample rate-to-bandwidth ratio
of just 2:1 is insufficient for reliable
digital signal measurements.