technology - When 2 witnesses are required to verify something, can one or both of them witness this using Skype?

Examples of when you need 2 witnesses testifying an event:

• groom's "purchase" / acceptance of the terms of the ketuvah


• witnessing kiddushin (groom giving bride a ring)

If two witnesses watch this event via video camera, Skype, etc. and are neither physically at the event nor is each witness in the same place as the other, is their observance acceptable testimony. Or must they be physically present at the event?

The 2 items listed are examples, The question is meant to be a general rule, unless certain types of witnessing have different conditions. If so, please state why.