Yes, you are looking at separate SPD for each half.The customer wanted it wiring this way so there are two mains units, one for the left half and one for the right half. I take it it would need an SPD on both units, I'm not even sure where the 2nd supplies comes from, its not looped from the other supply and looks newer than the original one.
If you had a common incoming point, such as an isolator switch feeding Henley blocks that then split to the different DB, you could place a common SPD there.
In all cases you need to keep the SPD cable loop length to 0.5m max (so L => SPD => E and L => SPD => L or N) for the SPD to be effective (see 534.4.8) as what impacts on performance most of all is inductance since the surges have most power at relatively high frequency compared to 50Hz power (around 100kHz). That is why your neighbour's SPD will do little to protect you even if it is on the same phase.
The inductance is also why you don't want coiled up cables feeding the SPD, and ideally you want to use high current, and C or D curve, MCB to reduce the magnetic trip coil's impact as much as you can. Or no MCB at all if the SPD is safe for your incoming fuse and you have an approved method of connecting it (such as neutral blocks in place of MCB to a busbar, or like the Hager kit that links across the incomer's connections, etc).