It sounds like you're talking about an oscilloscope or something similar. In that case, the channel-to-channel isolation describes the isolation between each channel and every other channel. The channel-to-ground isolation describes the isolation between each channel and ground. These are different, but related, concerns.
Channel-to-channel isolation describes how much voltage differential there can be between two inputs to the scope.
Channel-to-ground isolation describes how much voltage there can be between any input and the ground input to the scope, usually through the power connector.
The "techniques" involved aren't different at all. When two things are to be isolated, make sure there aren't any conductive paths between them, and make sure that all barriers between them (optocouplers, transformers, air distance, creepage distance, insulation, etc.) are rated for the isolation voltage you want.
Now, if all channels are isolated from each other, they must also all be isolated from ground; if they were all referenced to ground, they would all also be referenced to each other! It is, however, possible to have all channels isolated from ground while still being referenced to some other common node. This scenario can be useful, depending entirely on what it is you want to measure. If all the signals being measured are referenced to the same voltage, which is not earth, then your scope also having isolation to ground but not channel-to-channel would be fine.
As for crosstalk, that would require more details about what you're doing. I don't think there's a general answer there.
Leakage currents at high voltage but tiny current are common with older equipment and equipment with EMI filters. To answer all your questions, connect a 100k resistor between the two grounds, and measure the voltage again. If it's a lot lower, then it is a small leakage current, and they are safe to connect together.
Best Answer
"Isolation" means that a signal on channel 1 won't appear on the trace for channel 2 or vice versa. It doesn't mean that the signal input connections for channel 1 and channel 2 are isolated (although on some equipment this may be so).
An isolation of 100:1 likely means that a 1 volt signal on channel 1 might produce a 10 mV signal on the trace of channel 2.