You can do it something like this:
var condition1 = SearchSyntaxHelper.GetFieldCondition("_site", SiteContext.CurrentSiteName.ToLower());
var condition2 = SearchSyntaxHelper.GetFieldCondition("_culture", cultureCode);
var combined = SearchSyntaxHelper.AddSearchCondition(condition1, condition2);
Bear in mind you may also need to add SearchSyntaxHelper.GetRequiredCondition
which adds additional plus symbol (+) into Lucene search request that means it's a required condition:
var condition1 = SearchSyntaxHelper.GetRequiredCondition(SearchSyntaxHelper.GetFieldCondition("_site", SiteContext.CurrentSiteName.ToLower()));
var condition2 = SearchSyntaxHelper.GetRequiredCondition(SearchSyntaxHelper.GetFieldCondition("_culture", cultureCode));
var combined = SearchSyntaxHelper.AddSearchCondition(condition1, condition2);