The bitmap is per-array, not per-device. When you re-add a device, it will resync everything that has changed since the array was last completely in-sync and not degraded.
So you would need to plug both backup drives in at the same time.
If you create e.g. md0 from 'internal' and 'external-1', and then create md1 from 'md0' and 'external-2', and make the filesystem on 'md1', then re-adding either external device to the appropriate array will cause a resync of only the things that have changed since the last resync with the device. However that will require twice as many bitmap updates so writes can be expected to be slower (though making the bitmap chunk size larger might reduce this pain somewhat).