Marketo Measure - subchannel part of several channels?

Yes, it's completely okay to have multiple rules for one channel/subchannel. So e.g., you can (and should) have separate rules for Google as a sub-channel for both Paid-Search and Organic-Search channels, that's completely normal. Furthermore, you can also have multiple rules for a single channel and sub-channel pair too. Multiple rules can be thought of as “casting a wider net” to ensure that each touchpoint is mapped correctly. Often parameters can be incorrectly added or missed completely, therefore having multiple rules to capture a channel/subchannel is a good idea to ensure mapping accuracy. Keep in mind that Marketo Measure logic prioritizes touchpoint mapping in descending order starting with the top row of the spreadsheet and making its way down. Marketo Measure reads each rule (row), looking for the true and first fit. The touchpoint is then mapped to that channel/subchannel.