A couple of pointers to begin with, plus a warning/disclaimer.
The warning first, It's virtually impossible to delineate planetary placements on their own. There are lots of potential meanings to each planetary placement, depending on sign, house and aspect (the first two being especially important). As you haven't said anything about their house placements all comments will be of problematic value.
Many modern practitioners fail to distinguish between planets, signs and houses to any marked degree. So you will find statements to the affect that Mercury = Gemini = Third House. That might help in early studies but it creates a lot of 'unlearning' to do later. For example by the same reasoning Mercury = Virgo = Sixth house. A moment's reflection tells us the obvious conclusion of such shallow analysis is that Gemini = Virgo, which even a total novice can see to be rubbish.
A better approach is to realise that it's the planets that are the actors - they are active. Signs and houses are passive, they simply set the stage and environment within which the planet acts.
So let's take Mercury. Yes Mercury is to do with communication, and indeed the rational or logical mind - the nature of our explicit thought processes and the way we express them. (You will find that Mercury can mean other things as well but for the moment we'll keep to the best known dimension of Mercury).
Mercury in Scorpio tells you something about how your express this rational mind. Traditionally a planet's strength was essentially dependent on it's sign.
Mercury is not particularly strong in Scorpio (Mercury doesn't particularly work well in Water signs), However other factors may come to it's aid here. Without the chart it's impossible to pick them out but using my own chart I also have Mercury in Scorpio. It happens to fall in a part of the sign where Mercury does have some, albeit not much, strength. My Mercury also has a mutual reception with the Moon, by Triplicity. That is Mercury is in a Water sign, which the Moon rules by night (and I was a night birth) and the Moon is in an Air sign, which Mercury rules by night. The two planets are taken to collaborate or give strength to each other. Now it's not too important you fully undersatand all this now, my point is that a full judgement requires an examination of the whole chart.
Taking Saturn as the other planet you mentioned. Saturn is concerned with structures in all senses of the word, thus it governs any container, such as your body and its skeleton, it rules fences and walls, it rules plans and it rules methods of structured thought, such as the scientific method. It's Saturn that indicates the limits to your abilities, it provides the order and structure to your life. A Saturn that functions well, provides the base from which you can succeed, a Saturn which functions poorly may provide low quality plans or self limitation through, say, fear of the consequences.
Saturn in Gemini, is in an Air sign. Now Saturn rules the Air triplicity by day (and Mercury rules it by night), so if you are a day birth, Saturn is strong in Gemini and that should help your powers of structured communication.
Now that's only a brief sketch, the house placements of the two are very important, as is their relationship to other planets and other properties they may possess, such as being retrograde, or swift in motion, or their relationship to the Sun.