config.c umb_init expects to be and is called only once

[ecm-pushbx]

This is where config allocates XMS UMBs and sets them up as DOS UMBs:

kernel/kernel/config.c

Line 507 in d073436

STATIC void umb_init(void)

There are some use cases where two different device drivers will provide different UMBs. The canonical example is loading UMBPCI then subsequently loading a memory manager like Jemm or JemmEx.

Other DOS versions usually support allocating additional UMBs after subsequent device drivers ran.