Edit: What's below is not valid anymore because the OP has change the layout ... :-þ
You have 30 keys on the left-hand-side. That would fit perfectly into a matrix of six columns and five rows if one key on the Shift-row (which has seven keys) would belong to the bottom row (which has five).
You would need 6+5=11 wires in the cable between the halves if you'd use only a single controller on the right-hand-side.
With two controllers, you would need four wires (I²C + power) or three wires (USART + power). A four-contact 3.5 mm plug ("TRRS") is common for I²C between halves of split keyboards, but only because it is small and because cables are common. It is actually not very reliable for digital signals: it is not hot-pluggable, often not shielded and there can be noise if you turn it.
The alternatives to having another controller is to use a:
* I/O expander. A simpler chip with GPIO lines and an I²C connection. Four wires. However, not much cheaper than a microcontroller these days.
* A shift register IC for strobing columns. Typically has 8 outputs. Needs 2 wires + power, but would only handle columns, so you would only win two wires (or three if you change the matrix to 8×4).
However, you should also consider how many GPIO pins you would need in total, and how easy it is to hand-wire it.
The common Pro Micro has only 18 GPIO pins.
The right-hand side would be easiest to wire as a matrix of 9×5, and with a single Pro Micro, the full matrix becomes (6+9)×5, which would require (6+9)+5 = 20 GPIO pins. If you'd assign two keys from the top to the bottom row on the right-hand-side, you could do 8×5, and (6+8)+5 =19 GPIO pins.
With two Pro Micros, the right-hand side would need only 9+5 GPIO pins for the matrix + 2 for I²C = 16 pins (leaving two GPIO pins that you could use for LEDs or a rotary encoder).