Bi turbo and twin turbo are the same thing.
Twin turbo means two turbo's of the same size operating at the same time regardless of configuration (i.e. separate banks such as s4, or combined such as supra and rx7)
Supra owners often change the sequential setup to a "true twin"setup (search ttc mod on mkiv.com) which makes the turbos spool together. The rx7 has a similar mod but it is more indepth unless you prefer the poor mans method of wiring actuator doors open.
The rx7 and supra have two identical sized turbo's operating in sequential format, (therefore they are not "true twin turbo" setups from the factory). In other words there is a primary turbo and a secondary turbo, and the first turbo (primary) is not smaller than the secondary. The initial exhaust flow is directed to the primary turbo and is slowly fed to the secondary turbo as throttle/rpm dictates, therefore initial lag is reduced. The second turbo operating in the sequential format comes "online" once the exhaust flow has reached a point to support the flow of both turbo's while being able to maintain positive manifold pressure.
Hope that helps