double penetration: either 2穴 or 二穴 would work fine for searches.
Gokkun: either ごっくん or ゴックン and every so often ごっきゅん if they're trying to make it sound cute. If you're doing a search for swallowing in general you'd be well served by adding terms like 「飲む、飲み干す、飲んじゃう、飲んじゃって、飲んでくれた, 精飲」 and other variations of "to drink".