If you can duplicate/imitate the scattering of the sound from the rack & components behind your left speaker with diffusion behind your right speaker, you may be all right. If not, timbral (tonality, coloration) and temporal (timing) differences between the left and right speaker may be audible.
Planars can use all the space behind them as possible, with 5’ a good compromise. 5’ allows for a 10 millisecond delay between the sound from the front of the panels and that from the rear reaching the listener’s ears (sound traveling about 1 ft./millisecond.). 10 milliseconds is about what it takes the brain to differentiate between two identical sounds reaching the ears from the same location (the speakers). 10ms results in the sound from the rear of the speakers being perceived by the brain as separate from the sound directly from their front. That reverse wave of dipoles is one of their characteristics dipole enthusiasts value in a speaker, as it (re?)creates depth and a large sound stage. Dipole users not wanting those characteristics are free to place absorption behind their speakers.
Some dipole owners lacking 5’ of space behind their speakers move the speakers out for serious listening, moving the speakers back when not listening.
