At the risk of being called a spotter, and wearing my shabbiest anorak, here we go. Yes, the Spey-engined FGR2 could use both 7th and 12th stage air for boundary layer control. However, if I recall correctly, the on-speed was only 3 kts slower using both, and it casued the TGT to rise a few degrees thus adding wear and tear on the engines. So, a Bleed Control Switch was used in the Isolate position to select 7th stage air only. I'm a little hazy here in that it could have been 12th stage only, but I think I'm right in saying 7th.
The Spey F-4 BLC air was originally provided by either the 7th or 12th stage bleed, depending on the amount of throttle required on approach, which affected the bleed-air pressure. However, early in flight test at Edwards we discovered that the switching point was close enough to on-speed that it was either too much or too little. As a result, the bleed source would switch back and forth, resulting in "uncommanded thrust changes in condition PA". It was apparently solved by eliminating the automatic switching feature. This was just one of the surprises that resulted from "putting a proven engine in a proven airframe".