When done well, four-handed massage feels like you are completely enveloped in sensation and can be quite a wonderful experience. To synchronize, one therapist takes the lead and the other follows, but it should feel like they're doing the same thing simultaneously. It helps if they have a similar style and "touch" and have worked together before.
Four-handed massage is trendy and has been added to spa menus at many resort spas. (Some even have eight-handed massage.) In that setting you are less likely to get therapists who have worked together before. For a more authentic experience, get it at a spa that specializes in Ayurvedic therapies, like Elemental Embrace in Canada.