Okay I'm going to try to answer this as quickly and logically as possible.
Aliens would HAVE to have gland to store this "spitting acid" in because of one very simple, basic fact: they do not have cheeks.
Their mouths are filled with gaps and spaces on the sides of their heads, and this makes a vacancy through which air can escape. Therefore, they cannot develop the force necessary to fire a large quantity of fluid from their mouths because they would not be able to maintain the necessary pressure.
They also would be unable to fire any fluid from their secondary mouths because they are simply too small to generate the quantity of acidic spittle which can be seen in Alien 3 and Resurrection.
Also this "acid spit" is most likely just that: a highly acidic enzyme or chemical. It is NOT the same as their blood because if it was we would see the same consistency and properties, which we obviously do not. In fact, if I recall correctly, their acidic spittle isn't even the same coloration, which further hints that it is a different chemical entirely.
Additionally, it is extremely pointless to possess any sort of offensive capability which directly saps off of an organism's circulatory system. There is simply no evolutionary benefit when a xenomorph can just as easily cut itself and catch up to its prey.
Therefore, it is reasonable to speculate that the acidic "spit" is generated in glands within the xenomorph's head, and does not draw off of its circulatory system. It is most likely not the same chemical at all.
"If anything in this life is certain, if history has taught us anything, it is that you can kill anyone." - Michael Corleone