wearing both of them,then you can notice that backpack is invisible,it's weired,I heared wearing backpack in arma 2 lower stealth value,anyway lowering stealth value because of backpack but being invisible is also weired.
so,lowering stealth value means doing this can lead you to be more detectable
ghille shit has large space to carry something,I mean capacity,even ghille suit has great camouflage ability,so wearing backpack must cause penalty by making backpack visible and lower stealth value that it can be spotted by AI more
and now,ghille suit covers whole body,and it conceals infrared ray much better,it means at least backpack must be visible to find via eyes,even FLIR can't spot them,and also balancing