Rather than trying to describe my beef with this idiom, I will give a bunch of successively objectionable examples. None of these are taken from real life.
As I see it, if (and when) both "if" and "when" are used, they should both describe the following statement.
(in an argument) If and when you admit defeat, I will be gracious.
This example is a perfect instance of this: "you" have not admitted defeat and it is not certain that you will, but "I" am promising to receive it with grace when it does happen. "I" am not offering to make an exchange (which is what "if you admit defeat, I will be gracious") would imply), nor am "I" expressing bravado (which is what "when you admit defeat, I will be gracious" would imply). The combination of circumstance and syntax produces a perfectly neutral conditional.
(planning a war) If and when this country is invaded, we must be ready to fight.
This is more or less equally good at expressing unbiased uncertainty, though I suspect the speaker really meant to say "We must be prepared for an invasion", and thus, "We must be prepared! (Because we are at war and might get invaded)".
(concerning a pregnancy) If and when we have a boy, we must name him after my father.
This has some false confidence leaking through, since with a pregnancy underway, whether or not the child is a boy is not in question (even if it is unknown).
(family planning in general) If and when we have a boy, we must name him after my father.
This is actually expressing an agenda, and has very little to do with conditional action. The speaker intends to continue trying for a boy until one is born. Indeed, no pregnancy is yet extant, so the pure "if" statement is at best wishful thinking, while the pure "when" statement is expressing false certainty about an event over which one has no control.
(preaching caution) If and when this country is invaded, we must be ready to fight.
This is worse than the 2nd example because the speaker is actually raising from nowhere the possibility of an invasion, and simultaneously elevating it to certainty. You, the reader, may have the intended reaction to the statement "If and when the dollar becomes worthless", which can probably actually be heard these days.
(estate planning) If and when I die, I want you to have everything.
This is silly: everyone dies. There is no "if" about it. On the other hand, in circumstances where untimely death is a possibility, it would be inappropriate to say "when".
(sarcasm) If and when I ever learn the meaning of life, I will let you know.
This is even sillier: there is no possibility of "when". Without the possibility of "when", the "if" also means something else: it is no longer a true conditional or even a counterfactual; such a statement can only be sarcastic.
Here's my question, then: what connotation is this idiom "supposed" to have? What, at least, does it usually have? Does the "if" or the "when" dominate or are they intended to be balanced, as I think? And finally, as the title suggests, what difference does it make if their order is reversed?
Best Answer
The idiom "If and when X, (consequence)" has the meaning "if X happens, then when X happens, (consequence)". In other words, the "(consequence)" will not happen unless X happens, and will not happen before X happens.
It is generally used when talking about not making unnecessary preparations for an event that may not occur:
Some people might complain that "if and when" is redundant, because the "if" carries with it a sense of "when", in that you don't know whether X is going to happen until it actually does. But it's sometimes possible to know to some degree of certainty that something is going to happen, before it actually occurs - and in fact this is often the situation where the phrase is used:
Simply saying "if they open fire on us" can invite the response "but we know they're going to, so we should fire now"; the when emphasises the speakers view that no pre-emptive action should be taken.
As to the difference between "if and when" and "when and if", the basic meaning is identical (as others have noted), but there is possibly a slight difference of nuance: the standard phrase is "if and when X", but inverting it to "when and if X" emphasises the if part, thereby putting more emphasis on the fact that it is uncertain whether X will actually occur.