If you are on one street service, what ever water volume coming into the property is fixed. If you use water in either unit there will be a reduction in volume to the other unit, if the service is either too small or corroded by calcium. The same can happen on the main line from the meter to the water heaters. If the line is corroded by calcium, then the volume of water you have will be limited by resistance and size. I have seen 3/4" water lines to the water heaters with a 1/4" inside diameter. When one person used water anywhere in the house, the person in the shower got a reduction in water supply. The first thing to try would be to find out if you have galvanized piping. If you do, then they're probably clogged. If you simply replace them directly from the meter, you will have the maximum pressure and volume available from the service.
If your service is bad (calcium or minerals), then nothing you do on the inside will help. You can test it by checking the volume of water at the meter. If you remove the meter and run the water into a 5 gallon bucket, you can time the filling. This will tell you your flow rate. Most homes should have at least 40 gallons per minute, minimum. But duplexes should have at least 60 gpm. Therefore a 5 gallon bucket should fill up in 7.5 seconds at 40 gpm and 6 seconds for 60 gpm. Pressure is not an issue, you have the same pressure at the end of a 1/8"th inch pipe as a 10" pipe, but a 10 inch pipe has a whole lot more volume. So a main water service could have 65 pounds pressure at the end and the outside diameter could be 1" but the inside diameter could be 1/4" and you'd have no volume, and you'd think there was no pressure.
Finally, if the plumber replaced the water heater, often they shut off the water at the meter. Many times old meter valves get stuck part way open. This effectively leaves the house on a reduced volume, because of the restricted flow from the valve. Old brass valves tended to break very easily. The worst part is you can never tell it happened, because it breaks on the inside. The stem for the valve turns up, just like its supose to (telling the plumber the valve is open), but the disk on the inside stays closed or partially closed. If you have disk type gate valves, have them replaced with full port ball valves.
Raymond VinZant Plumbing Prof.