If the tank is well insulated, and the thermostat can be cranked way up, it's probably cheaper/more space-efficient to install a tempering valve at the output (set to 120-130F) and bump the tank temp up to 140-150F. If the insulation on the existing tank isn't so great the standby losses will go up considerably though.
Otherwise, putting them in series is fine for additional capacity, putting them in parallel (with carefully plumbed symmetry) will improve flow. All else being equal the standby losses of 2 tanks will be more than a single tank of equivalent volume (imagine that! :-) ), and probably more than a single well-insulated tank running at higher temp with tempering valve on the output. If there's space for a 15-18 gallon tank it's probably better from an efficiency point of view to replace the existing tank with the biggest tank that fits and add a tempering valve at the same time "just in case", then tweak up the temp only as much as is necessary for showering.
OTOH, (as owners of tankless HW heaters know), the definition of how long a decent shower is can grow to meet the new improved capacity. ;-) (I've heard of one guy who put a timer switch on the power to his tankless to keep his teenagers from taking hour-long showers. ;-) )