If you are using genuine current-output c.t’s, then connecting them in parallel results in the currents adding (or of course subtracting if you reverse one of them - either with the direction of the primary (the cable) or with the secondary connections). If they are the voltage output type, then wiring in series will have the same effect. Both c.t’s must have the same sensitivity, or you’ll need to be inventive and you might need an additional burden for one of them, the aim being to make them appear to have the same sensitivity at seen at the input to your inverter.