im attacking a battlefield with 2 units, then in the showdown i play demacian stand do my units get +2

Radiance rules aren't published yet. This uses the printed card and current rules only, and it isn't a ruling on the new mechanics.

No — playing Demacian Standard during the showdown doesn't give your units +2.

Two things block it. Demacian Standard is a Gear with no Action, and by default cards can't be played during a Showdown State (Rule 343.1.a) — so mid-showdown is the wrong window for it unless something grants Action/Reaction. On top of that, its buff is a triggered ability keyed to the event "when one or more friendly units attack here." Your units already attacked; that event has passed and triggered abilities don't fire retroactively. Note it also only gives +2 to one of them ("give one of them +2 [M]"), not both.

The line that works: play Demacian Standard to that battlefield during your Main Phase, before attacking. Deploy lets you play it to any battlefield (even one you're about to attack), and it isn't recalled from a battlefield. Then move your units in to attack — the trigger fires as they attack and gives one of them +2 that turn.

This is my interpretation based on triggered-ability timing. No official FAQ exists for this specific interaction.