That is simple - as John wrote, it’s the right-most cable coming out of your main meter. Or if it’s easier, the brown cable going into the bottom of the Henley Block.
Is the SDM 120A your generation meter? If it is, then the cable for the second c.t. is the cable from the ‘Line’ Henley Block (the left-hand one) that goes from there into that consumer unit - the one containing the generation meter.
If it’s easier, you can of course put the c.t. on the equivalent Neutral cable. If you do, it must face in the opposite direction.
Having put your c.t’s on those cables, you will have, in OEM terms, a “Type 2” installation. CT1 is the one below the Henley Block, CT2 is the one above it.
Like many suppliers, OEM has been hit by the global semiconductor problems. Keep an eye on the topic AVR-DB: emonTx V4, new hardware in progress - #157 by whitecitadel pinned at the top of the ‘Latest’ page for any news.