А как насчет .And(
с .AnyIn(
?:
IMongoCollection<Model> collection;
var builder = Builders<Model>.Filter;
var result = collection.Find(
builder.And(
builder.AnyIn(m => m.Tags, new [] { "T1", "T2" },
builder.AnyIn(m => m.Tags, new[] { "T3", "T4", "T5", "T6" }
)
);