Since this is a common facility for your tenants, it should be installed in such a way that neither tenant is paying for the gas or electricity required for the washer.
If there is no common hot water heater in the building, then you should do the following:
Have an additional electric meter installed to supply electricity to the common areas of the building. Depending on local regulations and the size of the building, this may need to be a commercial meter.
If you want to use gas for the hot water and dryers, have an additional gas meter installed.
In order to supply hot water to the washers, you may want to investigate tankless water heaters which take up significantly less space than a tank. A small 100,000 BTU heater like this one can easily supply several washing machines.
An alternative route is to go all electric. Install a common electric meter as above, but use an electric on-demand water heater and electric dryers. Depending on your local utility costs this may end up being a lot more expensive than gas, though.