Even when posted flippantly, the old man in me feels impelled to point out that wearing two condoms would actually increase the chance of unwanted pregnancy as they are not designed to be worn like that and would rub together and split.
And, yeah, wear a condom is the simple, one size fits all answer to this thread.
I worry about how people reach university age thinking condoms "aren't very reliable". They are. They're tested incredibly rigorously. Used as instructed, pregnancy won't result. It's pretty much as simple as that.
I'm not proud (but equally not ashamed) to say I've had a number of said encounters at uni.
I'm not a father, and I've not caught anything.
Blah.