Can Mercury and Venus be square or opposite each other?

They cannot. Both stay within a limited distance of the Sun, and neither of those angles is reachable from inside that limit. A report listing a Mercury–Venus square is describing something else, or has gone wrong somewhere upstream.

Is Mercury sextile Venus uncommon?

Less common than a conjunction between them, certainly. It needs both bodies near the far end of their range and on opposite sides of the Sun at the same moment, which is a minority of days rather than most of them.

What does it feel like from the inside?

Mostly like nothing, which is the difficulty with easy contacts generally. Being able to say clearly why something is good does not announce itself as a talent; it registers as the way things obviously are, and it is usually somebody else who notices it first.

Aspect

Mercury sextile Venus

Mercury sextile Venus is as far apart as these two ever stand. Both are tethered to the Sun — Mercury on the shorter leash, Venus on a longer one — so the pair can never stand far apart. A square, a trine or an opposition between Mercury and Venus does not occur in anybody, and the sextile is close to the widest separation the two ever reach.

Getting two signs apart takes something particular. The two have to sit on opposite sides of the Sun, which means one of them is rising ahead of it in the morning while the other is setting after it in the evening. A morning star and an evening star, in one chart, at the limit of the range either of them has.

Venus is read for eros, magnetism, value, and for values, what you find worth paying for. Mercury for voice, communication, the way you frame what you do. This is taste with a vocabulary attached to it.

Far enough apart to be told apart

In most charts these two sit close together, often in one sign, and when they do it is hard to say where liking ends and describing begins. Somebody built that way praises what they like and likes what they praise, and from inside the loop there is nothing to see.

At this distance the loop opens. Taste and its expression come apart far enough to be handled separately: a person can hold an opinion and decline to voice it, or make a warm and competent case for something they are only lukewarm about. The second ability is worth naming, because it is a professional skill and a small moral hazard in the same breath.

What the easy angle supplies is that the two cooperate on request. Preference becomes explicable — not merely that a thing is good, but why, in an order somebody else can follow. Being able to say why is a separate faculty from knowing, and a great many people have the knowing without it.

Why it reaches money as well as love

Venus is read on two fronts that turn out to be one faculty pointed at different questions: eros, magnetism, value on one side, and values, what you find worth paying for on the other. Mercury's business is voice, communication, the way you frame what you do.

Put those together and the contact lands in the middle of anything made of persuasion. Writing that sells. Teaching that makes a subject attractive rather than merely clear. Criticism of made things. And negotiation, where knowing what something is worth and being able to say so without heat are two separate wins that most people only have one of.

The caution belongs here too. A case can outrun the feeling underneath it, and this pairing makes fluent advocacy cheap to produce. Charm available on demand is a real asset, and it is not evidence of conviction — including to the person supplying it.

What changes the reading

Which of the two is the morning star. Older practice reads a body rising ahead of the Sun differently from one setting after it, and modern practice mostly lets the distinction go; in this pair it is the variation that is always present, so it earns attention here that it does not always earn elsewhere.

Closeness, and it counts for more here than usual. This pair spends its whole life within a short distance of the Sun, so the band in which the two are far enough apart to register as a sextile at all is a thin one — and the margin the angle is granted is the narrowest in the scheme. A contact sitting at the outer edge of that is doing very little.

And the arena, which the angle says nothing about. Two charts can carry the same contact and one of them spends it on sentences while the other spends it on rooms, fabric or food. What a person has taste in is decided somewhere else entirely.

Questions

Can Mercury and Venus be square or opposite each other?: They cannot. Both stay within a limited distance of the Sun, and neither of those angles is reachable from inside that limit. A report listing a Mercury–Venus square is describing something else, or has gone wrong somewhere upstream.
Is Mercury sextile Venus uncommon?: Less common than a conjunction between them, certainly. It needs both bodies near the far end of their range and on opposite sides of the Sun at the same moment, which is a minority of days rather than most of them.
What does it feel like from the inside?: Mostly like nothing, which is the difficulty with easy contacts generally. Being able to say clearly why something is good does not announce itself as a talent; it registers as the way things obviously are, and it is usually somebody else who notices it first.