In my biochem course, we learned that GPCR receptors trigger a phosphorylation cascade, with the end result being a large amplification of the signal in the form of cAMP. We never studied any particular GPCR individually, but we were told that GPCRs always end in the formation of a large amount of cAMP, which will go on to phosphorylate key targets that achieve the end result.
My question is as follows: If there are two GPCRs on the same cell membrane (let's say GPCR-A and GPCR-B triggered by substrates A and B respectively), how does the cell "know" which GPCR triggered the phosphorylation cascade if the end result is the same (large amount of cAMP). In theory, couldn't substrate B bind to GPCR-B and cause a phosphorylation cascade identical to that of GPCR-A, thus triggering GPCR-A's cellular response?
Is there some deeper specialization/uniqueness to each type of GPCR receptor that we didn't cover in my course? Does each GPCR produce a slightly different cascade that ends in something similar, but not exactly cAMP? Or is there some restriction, like each cell limited to only one GPCR (I would find this surprising, if this is the case)?