For Conjoined Beams and the requirement for its additional damage output:[1]
Both Beldum and Metang have to be on the Bench; only having 1 of the Pokémon does not count.
Only cards explicitly named "Beldum" and "Metang" are counted. Cards whose names contain "Beldum" or "Metang" but are not named exactly those (eg. "Steven's Beldum") do not count.