CYBLE-012011-00/CYBLE-012012-10,EZ-BLE™ PRoC™ Module | Cypress Semiconductor on page 6 lists the keepout area for the antenna on the module. This should be enforced for both chips, and no metal should be placed underneath or above the antenna itself for least interference.
If you are able, I would personally recommend placing the modules on opposite sides of the board to minimize EMI between them; Otherwise, if due to board layout etc. you need to place them closer together, then I would try to follow the below rules in order of priority:
1. Adhere to the keep-out area for the antenna and don't place metal within this area (or underneath the antenna).
2. Place the module with the antenna on the edge of the board.
3. Place the modules so the antennas are as far away from each other as possible.
4. Place the modules so that the antennas are "facing" away from each other (180 degree rotation between the two modules so that the antennae are pointed away from each other)
5. Setup supporting components/circuitry to minimize EMI propagated through the power/ground wires/planes supplying the two modules. This is more important for the high-speed scenarios, and at 50 MHz might be negligible. For example, place capacitors between ground and power pins near each module, make data lines as short as possible, etc. These are pretty much general layout rules however, so you should already be using/know them.
thanks, I will try it like this. Perhaps I rotate one board to the left and the other one to the right, so I will not have a good and a bad one.
If you follow the keep-out area for the antennae then you can probably even place the two chips next to each other with the antennae facing each other, but....obviously that would give pretty high EMI even if the board design still works. It really comes down to how much EMI you can tolerate, and for how long; Some EMI will be momentary, some will be long-term. The combination will determine how reliable the communications channels are. Since the antennae are not really directional in design or operation, the things that will affect the EMI will be: metal (which blocks signals really well), PCB feedback (pulling power from the power source, for example, might cause a drop in voltage that propagates back through the wires into the input power of the other module. This is why people usually place Capacitors/Inductors on the power circuitry.), proximity (signal strength drops over distance cubed), and directivity (some antenna, for example radar dishes, are very focused in direction, and thus have a much higher signal strength in the direction of interest).
If you can, have the antenna on the two boards facing away from each other (one points left, the other right, etc) this will help I think.