The mutual interaction of two
radio antennas is not an issue. It is a natural behavior. All antennas would
work best if they were located to be free from surrounding conductors.
The effect of nearby conductors on an
antenna is increased if the nearby conductors happen to be of a length or configuration that causes them to resonate at the same frequency as the first antenna is operating at. In order to reduce interaction between two antennas with a mutual resonant frequency, the physical separation should be at least one or two wavelengths.
If two antennas are not mutually resonant and are resonant on widely separate frequencies, the effect of one on the other is reduced, and closer physical spacing can usually be tolerated.
Antennas for cellular telephones can be resonant over a very wide band of frequencies. Usually they are in the
radio spectrum above 500-MHz.
Antennas for WiFi are usually designed for much higher frequencies, some as high as 5,000-MHz.
Before assessment of how much interaction might occur between an antenna for "cellular" and an antenna for "WiFi", it would be necessary to know the frequencies involved.
Note that as frequency increases, wavelength decreases. An antenna designed for 5-GHz means one wavelength is only a little more than two-inches. If you have a one-foot separation of that antenna from another, it has about five-wavelengths separation.