The internal antennas work fine in my experience. As to being able for the 'other' captain to see the screen, that can be a challenge since many of the displays have limited viewing angles. The screen would have to be rotated towards the port chair. In addition, if either of you wear polarized sunglasses, small changes in the vertical viewing angle can make the screen appear to go blank.
If you share a screen between plotting and sonar, especially if you split a smaller screen, things are even harder to make even when looking head on.
If you can, take a look at friends setups. A trip to a decent marine supply store will also let you see what screens look like from the front and from the side.