you have a few options one is like you are saying add a second tank of equal size this would require piping in the hot and cold the same excat same way so that each nipple and 90 are the same length up to the tees where the wate renters the system and leaves the system
the other way ( the way i recomend ) is put in a rheem tank less unit the ones that make on demand hot water
remove one of the drain valves at the bottom of the water heaters and install a nipple then a tee
on the top of the tee set a brass circulator and off the run of the tee put in a drain again
after the circ install a va;ve and a drain cock just like a boiler
http://www.aaronsinspections.com/Water%20Heater%20Plumbing.pdf Here is a great drawing and description of water heaters installed in parallel. This is what I would recommend to increase water volume. http://www.askthebuilder.com/B413_Hot_Water_Recirculating_System_-_Installation_Tips.shtml Here is a website that offers information on recirculating systems.
You are in the right direction and when you install the return line and extra heater your hot water problems will be solved. I know it's a lot of information to read but between those two sites you have all the information you will need to install your system.
If my answer helps please click accept, If not I would be happy to continue to work with you and try to get the information you need.
You basically continue the hot water from the farthest fixture away from the water heater. You bring this pipe all the way back to the water heater and connect it to the drain plug at the bottom of the heater. http://www.us.grundfos.com/web/download.nsf/Pages/D6774D5BEBD2117B8825652200650E2C/$File/Uprec-gd.pdf Here is a site with a bunch of different hot water return installations. It isn't that complex in practice. You basically make a big loop with the hot water line, and use a pump to cycle the water through the line and back into the water heater. You have to insulate the entire hot water loop to minimize energy usage. Also make sure you install a check valve where the return loop connects to the water heater. You don't want cold water coming back through the hot loop.
Get a small sized pump and perhaps even put it on a timer. A small grundfos or lange pump should be more than sufficient to move water in the lines. If you move the water with too big of a pump for your application, you will subject your water lines to premature wear.