both units target each other using "Rampage". is it considered "in combat"

No — Rampage doesn't put the units "in combat."

Combat in Riftbound only happens when units Move such that opposing units are staged at a battlefield and the Combat resolves at Cleanup (Rules 452, 460–461). Rampage is a spell resolved on the Chain — it never stages a Combat, and neither unit gains the Attacker or Defender designation that defines being in a Combat (Rule 464.2.c). The units simply deal damage equal to their Mights to each other as a spell effect; that damage is not combat damage, so no Combat Cleanup or healing happens when Rampage resolves — the damage stays marked until the next Combat Cleanup or end of turn. If they're in an actual Combat Showdown when you cast it, they were already "in combat" for that reason — not because of Rampage.