Methods of decreasing the coupling between two closely placed antennas
Increasing distance between antennas would be the best option to lower coupling.
Can chose using chip ceramic antennas, which due to their small dimensions gives you more opportunities where to place in your wireless device.
https://www.johansontechnology.com/d...50AT45A100.pdf
If it is near field, I would think about 90° rotation to reduce E and H near field coupling.
Patterning such as electromagnetic bandgap structures (EBGs) and defected ground structures (DGSs) have been used for this purpose.
Distance is the most practical method but you posed the question by saying the two antennas are close. I have often addressed this issue and found "solutions" after closely examining the situation with a simulator that allowed one to view the local field structure. By looking at the SIMULATED fields one can often get an idea of what to try and where coupling is occurring.
Fences and slots can sometimes be helpful. If the frequencies allow effective RAM or absorbent materials sometimes that can be helpful. Resonant notches and chokes can sometimes help. Polarization adjustments, mentioned above, can also sometimes help. If there are ground planes in the mix modifying the shape and dimensions is sometimes helpful.
You asked for general guides. I find that decoupling in the -20 to -30 dB range generally to be realizable. -30 dB to -60 dB is significantly more difficult and Is now getting harder to measure. Decoupling more than -60 dB, good luck. By the way, a broken connector looks very much like high isolation. Keep an eye out so you are not fooled. Scattering from nearby structures starts confusing measurements around -50 dB and -60 dB.
People regularly try active cancellation with much cost, complexity and some success.
Are you close both in parts of a wavelength and physical distance? My point is that as frequency increases the electrical separation increases for two fixed antennas and sometimes helps the situation.
Azulykit
Another thought:
There are only four possible connection mechanisms. Direct conduction (a wire), capacitance, inductance, and radiation (RF). Direct is strongest and RF is by far the weakest. As you look for ways to increase isolation keep this in mind. Capacitive and inductive coupling generally decreases with 1/separation. RF coupling falls off at 1/(separation squared). You might also find that the coupling between the two antennas has locations where decoupling is enhanced while still close. If you have some positioning latitude that sometimes helps. Keep an eye for coupling via nearby cables and structures. If you have marginal matches the cables can become "hot" and couple. Run a hand up and down over your feeds while watching the match to see if you have anything like that going on.