What do you mean by events in one frame being simultaneous with the events in a different frame?
Two events can be simultaneous (or not simultaneous) within one frame, not across frames.
Two events are simultaneous in a frame if their t coordinates in that frame are equal. For example the two light sphere events
A : (xa = cT, ta = T)
B : (xb = -cT, tb = T)
are simultaneous in the frame O where they have the above coordinates, because
ta = tb
But in the frame O' where they have the coordinates
A : (xa' = ( cT-vT)*gamma, ta' = (T-vT/c)*gamma)
B : (xb' = (-cT-vT)*gamma, tb' = (T+vT/c)*gamma)
they are not simultaneous, because
ta' <> tb'
You can then swap the apostrophe annotation my formulas, so the events are simultaneous in O' but not O.