It seems there is no convention for naming twin or bi turbos.
Just to say that 'twin turbo' is most commonly a turbo for each bank of cylinders. Bi-turbo is no doubt a gimmicky phrase. Similar to 4wd vs. Awd.
Sequential twin turbos are usually called just that, sequential, to differentiate them from the more common twin turbos.
In the case of the Liberty B4, the sequential is employed, but there is apparently a noticeable 'gap' in power, at the peak of the smaller turbo before the larger kicks in.
Properly tuned, a sequential turbo can greatly *reduce* lag, but turbo lag is caused by having a turbo. Period.