Also, can you double up plugs in a larger hole ?
.
Having worked in a tire repair job for a while and having gone 4x4'ing and seen several cords used the answer is NO YES NO
No you should not ever use more than one repair cord.
Yes you can use more than one IN AN EMERGENCY!!!!! The Tire is basically going to be hooped because a hole that requires more than one repair cord has MOST likely caused the tire to sustain SUBSTANTIAL cord damage and should not be driven on they highway as it is likely no longer safe and if it is it should be done at a REDUCED SPEED!!!!!
No do not do this unless it is absolutely needed to get you to a place where a replacement tire can be had due to the previously mentioned potential for cord damage. OR AT THE LEAST checked by a qualified person at a reputable service or tire repair shop.
A tire repaired with a single cord should be take off the rim ASAP and booted with a patch as well this is the best method for repairing a tire. When you can lube the cord with a good coat of patch cement and allow it some time to cure before travel. This helps to ensure (DOES NOT GUARANTEE) that no moisture enters the actual tire carcass and subsequently cause degradation to the belts in the tire.
Did I mention Check the spare for Air before leaving home and ensure you have a jack and wheel wrench too!!! Wheel wrench must fit lug nuts on your vehicle....