Lots of great advice from others. I am going to avoid specific recommendations, other than cautioning you that with plumbing parts, "you get what you pay for". Cutting corners just means more repairs in the near future, so it is not worth it.
Water usage is very much a byproduct of number of people in the property. Showers, toilet flushes and laundry all increase with more people. Keep that in mind if you split billing. Even if the unit sizes are the same, the number of occupants will affect how much water they use.
Along with instituting changes to reduce water usage, I would also educate your tenants. Explain to them that taking long showers or doing small loads of laundry can drastically increase water usage and therefore increase your costs, which will increase their rent. In other words, help me help you!
For water usage there are several areas to look at:
1. Toilets - make sure you have low flow 1.5 gallon or less per flush. Older toilets could be 5-6 gallons! There is a number right on the toilet if it is is low flow. Many cities have rebates to upgrade older toilets.
2. Showers - install low flow shower heads. Buy a decent shower head so you don't compromise quality, otherwise the tenant will just replace it.
3. Washing machine - this is a major water consumer. Not a big issue if you have coin operated, but is a big deal if you are paying. Not only do tenants do their own laundry, but sometimes they have friends or family over to do laundry! Prohibit that in your lease. Low water usage washers are well worth the money. They are usually the front load type.
4. Outside water - people sometimes overlook the outside water usage. This can be sprinkler systems, which are a major water user. It can also be outside spigots. Tenants will connect hoses to wash cars or fill swimming pools. You can install locks on outside water spigots and that will prevent them from being used.
5. Inside water faucets - generally these are only used for brief periods of time. Any newer faucets have aireators that reduce water flow or you can replace them with low flow aireators.
I would recommend doing a full audit of water usage and make necessary updates / changes to reduce consumption. Provide a monthly water usage summary to tenants along with tips to reduce consumption. The more you remind them to change their behavior, the more likely they will make changes.
