The WattsOn is capable of everything the emonTx can do, plus much more.
From the manufacturer’s product page:
WattsOn provides comprehensive per phase (as well as cumulative) information, including Volts, Amps, Real Power, Reactive Power, Apparent Power, Watt-hours, VAR-hours, VA-hours, Power Factor and Frequency.
You’d need only one. The WattsOn is capable of three-phase, as well as split-phase operation.
Here’s the app note that shows how it can be used in the manner you’re asking about:
AN0306-WattsOn_Split_Phase_Inverter.pdf (43.5 KB)
The AN describes a 240 Volt “generator” (e.g. a PV system) across both hot legs of a split-phase system.
If I understand your single phase system correctly, you’d need only two CTs
vice the three shown in the app note diagram.