A divided tank is a great space-saver, but bettas can still see and flare at each other through most dividers, which causes constant low-level stress even without physical contact.
Use an opaque or heavily planted divider rather than clear mesh if your bettas are flaring often. A little visual contact is fine and even enriching, but constant flaring burns energy and can suppress appetite over time.
Check flow on both sides after installing a divider — filter output can create a dead zone on one side and a strong current on the other. Bettas are weak swimmers relative to their fin size, so a corner with no current for resting matters as much as filtration.
Give each compartment its own decoration and resting spot near the surface. It’s a small tank real estate, but bettas that have a den to retreat to settle in noticeably faster.