Selecting a GTM pool member based on the status of another gtm pool member

I believe there is already an inbuilt functionality called depends-on,

depends-on                    Specifies the names of virtual servers on which this pool member depends.

Try the below,

tmsh modify gtm pool b-pool members modify { memberB1 { depends-on add { memberA1 } } }
tmsh modify gtm pool b-pool members modify { memberB2 { depends-on add { memberA2 } } }