Repulsion of magnetic fields
Magnetic fields created with electrical currents in two coils placed according to the Helmholtz configuration will create an even field between these coils. This requires placing the coils exactly in line with each other and that the distance between them is exactly half of the diameter of each coil and that the magnetic fields are in the same direction.
See extensive explanations on the Helmholtz coils page.
When 2 coils are placed opposite of each other and their magnetic fields are opposite they will repel each other and create a completely distorted magnetic field. This field might even be outside of the intended area where the coils are placed.
Battery powered PEMF devices are usually supplied with 2 small coils, each with a diameter of 2 inch (5 cm). In order to obtain a homogenic magnetic field these coils will have to be placed exactly in line with each other and exactly with a distance of 1 inch (2.5 cm) from each other and with the magnetic field in the same direction.
In practical PEMF applications this is simply impossible although some manufacturers tend to advise to place the coils opposite of each other when treating e.g. a knee or a shoulder.
Not only there will not be a homogenous magnetic field between the coils but even worse because we do not know if the fields are in line or opposite with each other, possibly distorting the magnetic field completely!
Smaller handheld PEMF devices are intended for use on localized areas of the body however for the best possible results with pulsed electromagnetic fields applied to localized body areas, the use of only one larger sized coil will obtain superior results.