ESP32-Cam Serial U0Rx causing boot failure - multiple devices

[HardyM]

HI
I have a simple circuit with 2 x ESP32-Cams. They talk to each other with U0Tx -> U0Rx interconnected both boards.
One board (board1) also has an SD card inserted (using 1 bit mode, so IO4 is disconnected from the SDcard carrier)

When I power up the circuit, board 1 does not boot by program unless I disconnect it's U0Rx from the board 2

Boot of board 2 is not affected in the same way, and switching the two boards makes no difference.
Also, they both run the same code.

I've tried a number of these boards,. including some brand new ones.

Now - saying it doesn't boot isn't 100%. It gets this far.

rst:0x1 (POWERON_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
configsip: 0, SPIWP:0xee
clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00
mode:DIO, clock div:1
load:0x3fff0018,len:4
load:0x3fff001c,len:1216
ho 0 tail 12 room 4
load:0x40078000,len:10944
load:0x40080400,len:6388
entry 0x400806b4

Thats where is stops.

A good boot looks like this

rst:0x1 (POWERON_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
configsip: 0, SPIWP:0xee
clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00
mode:DIO, clock div:1
load:0x3fff0018,len:4
load:0x3fff001c,len:1216
ho 0 tail 12 room 4
load:0x40078000,len:10944
load:0x40080400,len:6388
entry 0x400806b4
Number 5 is alive
SD_MMC Card Type: SDSC
SD_MMC Card Size: 1910MB
Total space: 1909MB
Used space: 0MB
....and so on.

What exists in this preboot-loader that stops my program from running ?
Anyone know ?

I should add that if I disconnect the Rx line, power up, it boots. Then I can connect the Rx line back to board 2 Tx, and they chat together like best buddies.

Thanks
Hardy