What happens if I sign more than one contract and make my decision later?

A lot of these questions answer themselves if you flip it around.

"Our company sent an employment offer out to Pnatk and SuperiorCandidate, figuring only one would agree. Turns out both accepted and signed, but we only want one. How do we tell Pnatk that he's not hired after all?"

Yeah, that company sounds like a real a-hole entity, doesn't it? Why would they send a written and official offer... and then turn around and try to revoke it? Even if it was somehow entirely legal, it's still a terrible thing to do. You wouldn't want to ever work at a place like that

And that's what you're thinking of doing, Pnatk - and every company would feel the same way as the example above (and would want nothing to ever do with you ever again.) You accept an offer and sign a contract to work with a company? Then honor your agreement.

Recently I was in the position as you describe. Having a concrete offer/contract of company A available while awaiting the decision of the much larger and more bureaucratic organization B. Being soon out of a job that time and being the only breadwinner of my family I decided to play safe and accept the offer of company A. However about two weeks later organization B finally came around with an offer that was just a lot better (for me) in many ways. Although I felt slightly guilty about it, I accepted the offer of B (where I still work now) and canceled A. In my case it all ended without drama, lawsuits and/or angry emails/phone calls. The owner of company A even ended his last email to me with a polite success-in-the-future.

So based on my own experience I think it is a viable strategy if you can't afford to decline a job but still want to keep an option open for another job. However I think it is better not to pull this trick too often and only use it as a last resort.

Important detail in this story is that the contract of company A contained a probation period of a month which enabled me to back out of it in my jurisdiction, the Netherlands. IANAl but I think that having a probation period in a contract will enable an employee to legally back out of the contract beforehand in most other jurisdictions as well.

Most companies will just blacklist you, assuming that taking you to court for damages and/or breach of contract is not cost effective.

German companies would very likely take you to court regardless of cost, because that kind of thing pisses them off, it is dishonourable, and they will try to make you pay for this dishonourable behaviour. Other countries or just companies might do the same.