If 2 arithmatic layer happens in same time

Both effects still apply — and each applies only once. Order in the Arithmetic layer is decided in three steps: first Dependency (CR 480) — if one effect changes what the other does, the independent one applies first and the dependent one second (e.g. the example in CR 481). If there's no dependency, timestamp order (CR 482) applies, earliest effect first.

For a true same-timestamp tie, Riftbound's rules don't define a tiebreaker (the FAQ flags this as a known gap), but two sub-rules still do work: within the Arithmetic layer, increases are applied before decreases (CR 479.4.e), and in practice, if both effects are plain +X/−X bonuses, the order doesn't change the final number — order only matters when a min/max limitation or a dependency is involved.