Typically you will need to have as much distance between each antenna as possible. The antennas will need to be placed around the perimeter of the board and away from things such as batteries, displays and taller metal components like RF shield cans and connectors.
If you are using two LTE antennas they should be mounted 90 degrees apart from each other to maximize the reception of different polarized signal (called orthogonal diversity) and at least 50mm away from each other (called spatial diversity).
You should also consider that some antennas work better on the corner of your board versus others that work well in the center of the long or short edge of the device.
Some types of plastics will also dampen your signal such as glass filled nylon so keep this in mind when tuning the antennas in your chassis as this will also tend to shift the resonant frequency of the antenna. It is a good rule of thumb to always tune your antennas as close to the environment that they will operate in.