it depends on the number of others at the table, mainly
All you really do is reverse the equations you use when playing multiple hands in + counts. You have a negative segment of a shoe that consists of N cards. At an average rate of card consumption per round it will cost you how much $ per card dealt to get through this segment.
If heads up, you can increase your total action by as much as 50% and consume cards for the same cost, 2 hands of $15 rather than one hand of $20, for example. Suppose you have a segment with a -1% ev that is an average of 16.2 cards. With an average of 2.7 cards used per round, 5.4 total for you and the dealer when heads up, it takes 3 hands to eat up the negative segement. At $20 per hand on one hand the three rounds gives you $60 of total action exposed to the 1% negative ev to get through this segment. If you play two hands then a total of 8.1 cards per round are used, with a total of $30 in action per round. You get through the negative segment in two rounds for the same $60 of total action. The ev is the same. If you feel like you can get through two rounds of two hands faster than you can three rounds at one hand then there is a time savings. I believe there is a time savings for multi-hand in face up shoe but not in pitch.
As more players are added to your table the value of multi-hand in negative counts decreases and the harm you normally cause to yourself by playing multi-hands in + counts decreases as well. You have others to eat your negative cards while you can eat up their positive cards.
When one's spread is very big, you can sometimes gain even by playing multi-hands in negatives with twice the action you would bet on one hand. This occurs when the actual value of a card (or 2.7 cards) consumed becomes greater than the negative ev. For example, a backcounter could calculate a value for each card he observes (if there was no value to counting them then he would obviously not be doing it). When the negative ev on a hand played is so minimal that it is actually less than the gain for seeing the cards then it is of value to play the additional hands. Example, you are backcounting a game with only one player and it is going very slow. You would gladly stick a penny on each of the other six spots on the table just to see the cards dealt because the value of seeing them far exceeds the negative ev of your one cent bet.
Because of the complexity and the can of worms it can open, I think players will do well to simply simulate their intended strategy on SBA or something and look at the results for various numbers of players at the table and playing various numbers of hands at different counts. To get a fair comparison of hourly win rates, which some are not all that concerned with, one needs to assess an estimate of play speed (rounds per hour) associated with their single and multi-hand strategies.