One hand vs. two

Should a player always just play one hand at a time? Are there situations such as table conditions, number of decks dealt, etc. where two decks are more advantageous? I am especially interested in opinions as it relates to one deck play. When I play at a table with the rule of 7, where I get 5 deals for two hands, by the time the game is finished, the dealer usually only has about 8 or so cards left in their hand. Whereas, when I play with one hand with 6 deals, the dealer has about 18 or so cards left over. It appears that that last deal with the two hands would be especially advantageous for the card counter. Please let me know if there is specific written material showing an analysis of one hand vs. two. Thanks a lot.