I wait until mine are about 12 - 14 weeks before putting them outside.
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great infoSince there are only two of them, their ability to snuggle into a warm chick raft is severely limited. I'd continue to provide a heat lamp until they are fully feathered on heads, backs, and necks. It's not a big deal for them to have bare sides right under the wings since that area is covered well (and is the last to feather). Most chicks are plenty feathered to do without a lamp at 6 weeks, some as early at 4 or 5, some a bit later. You can push it more when there is a larger group to snuggle together but for two, I'd err on the side of more feathers is better.
FWIW, you can put them outdoors at any age (yes, even day-olds) as long as you provide heat, draft protection, and a secure coop. My brooder was outside from the very start in spring when temps were somewhere between 35 and 80 for the duration of their heat lamp stage. The brooder was large enough and ventilated such that the temperature away from the lamp was very near the temp of the outside air but the sides were tall enough that it wasn't breezy inside even when it was windy outside. They all thrived under those conditions.