md: Autodetecting RAID arrays.
md: autorun ...
md: considering sdb1 ...
md:  adding sdb1 ...
md:  adding sda1 ...
md: created md0
md: bind<sda1>
md: bind<sdb1>
md: running: <sdb1><sda1>
md: kicking non-fresh sda1 from array!
md: unbind<sda1>
md: export_rdev(sda1)
raid1: raid set md0 active with 1 out of 2 mirrors

The md0 raid kicked sda1 out because either I never synced it before rebooting or it was "halted" because md1 was rebuilding.

The solution is just to add it to the array so it will sync

mdadm --manage /dev/md0 --add /dev/sda1

After it syncs you'll see it shows both disks as active and up:

md0 : active raid1 sdb1[0] sda1[1]
      307202816 blocks [2/2] [UU]

As you can see below, md1 was just fine:

md: autorun ...
md: considering sda1 ...
md:  adding sda1 ...
md: md0 already running, cannot run sda1
md: export_rdev(sda1)
md: ... autorun DONE.
md: md1 stopped.
md: bind<sda2>
md: bind<sdb2>
raid1: raid set md1 active with 2 out of 2 mirrors