Can i use “if and only if” and “then and only then” in the same sentence for emphasis?

I want to be crystal clear and add much emphasis that something will happen if and only if a specific condition is met. I thought of If and only if and then and only then.

Can i use both in the same sentence to give emphasis to the condition that must be met?

Example: "If and only if you are wearing your spacesuit, then and only then can may you exit the spacecraft."

Would a native speaker express in such a way to give emphasis or that would sound unfamiliar in English?

• (i hope i will get an answer from a native speaker, NASA wants their door sign ASAP!) – Sharky Mar 16 '17 at 19:33
• "then and only then" would need to continue ... "can you exit". In my head, at least... – Catija Mar 16 '17 at 19:35
• @Catija fixed! thanks! well anybody, feel free to edit – Sharky Mar 16 '17 at 19:35

I'd say that it's OK. There's no grammatical rule that says you can't do this. I'm guessing that I've seen it done before - though I'd guess it's done more for comedic effect than in a serious way. I'm also pretty sure that I've seen someone say it in a comedic film, too.

I might make the suggestion to set off the "and only then" with an em dash or commas... this will make the stress of "and only then" even stronger.

If – and only if – you are wearing your space suit, then – and only then – may you exit the space craft.

If you really want to make it official, you'll want to use "may" not "can"... they certainly can leave the space craft without a space suit but it wouldn't be a smart idea.

• In suitable atmosphere, leaving without spacesuit wouldn't hurt. – Hector von Mar 16 '17 at 20:10

You can, sure, but the distinction is redundant. At least in mathematical logic, "iff" is used alone for the equivalence relation. If the condition is vital, the repetition might distract from the message.

After all, if and only if is redundant, already.

• strangely redundant? my goal is to give emphasis. would a native speaker express in such a way to give emphasis or that would sound unfamiliar in English? In my native language it sounds pretty normal but i have doubts about this in English, hence the question :/ – Sharky Mar 16 '17 at 19:28
• I wouldn't know, I am not a native speaker. Haven't heard it though and I guess that's because it interrupts the flow of speech. – Hector von Mar 16 '17 at 19:31
• On the other hand, it might be the logical next step after if and only if, which is already redundant compared to only only if. – Hector von Mar 16 '17 at 19:40