A parallel circuit has each load connected directly to the source. A series connection has the current flowing through each load, end-to-end. The heaters need to run in parallel. If you did run them in series, they'd put out about half the heat since the resistance would go up, the current flow goes down.
If the thermostat is rated for 20A, you should be able to put up to a 20A load on it. It would likely last a little longer if the load was less, but not necessarily so.