No. Most marine GPS antennas are active antennas, which means they will have a built-in LNA (low noise amplifier) to improve the signal to noise ratio of the incoming GPS signal. This LNA requires power which is obtained from a DC source on the device it is connected to. By connecting it to two devices that are each attempting to supply power you run the risk of causing damage to one or all of the devices. There are other complications that will inevtiablly arise too (e.g. noise introduction, signal loss) so it is highly recommended to avoid this approach.
The WatchMate 850 , WatchMate Vision and XB-6000 have an internal GPS antenna built in. So if your installation will give the WatchMate a clear view of the sky then you may not need an external antenna at all. If you need an external GPS antenna, ensure compatibility and maximum performance by using a Vesper Marine External GPS antenna. The use of any other antenna is not recommended.