Our application has two extremely chatty peripherals connected to UARTs. We used to use UART#1 and UART#2, but had a lot of problems due to known hardware FIFO reset issues. We fixed this by moving the MOS debug onto UART#2, and using UART#0 and UART#1 for our application. We used to do this by setting MGOS_DEBUG_UART to 2 in the 'build_vars" section of mos.yml.
Recently though, (since 2.7.0 I think, but cannot be exact) this has stopped working. I find that I now also have to manually edit line 22 of the file deps/modules/mongoose-os/fw/platforms/esp32/src/esp32_src.mk from
MGOS_DEBUG_UART ?= 0 to
MGOS_DEBUG_UART ?= 2 for this to work -- IOW, it looks like the mos.yml version is overwritten at some point.