Do one of them.
That’s it. No great debate, no big decision, just do one of them. Either of them. Doesn’t matter which. Try random, try flipping a coin, try alphabetical order. Just try something, just pick something, and start on that right now.
In the most extreme case I can think of, you have two commissions from different places. Both equally important, equally urgent, both equally on deadline, both equally hard to do. And you can’t afford to disappoint either commissioner.
Even then, do one.
The odds are that you won’t really disappoint anyone: even if the other piece is a bit late, often that’s okay. But even if it isn’t, even if choosing to do one commissioner’s job will seriously disappoint the other, you should still do it. Because the alternatives are worse.
The first and most obvious alternative is that you will try to do both pieces in the same time. We can do amazing things under pressure but even if you manage to finish both pieces, neither will be good enough. Cue disappointment from both commissioners and the certainty that you won’t be hired again.
But then the second alternative is that you try being clever. Clever is good. Except in this case you try to prioritise the pieces, looking at which commissioner will be the most valuable to you later, which job is the better. But we’re talking about the most extreme case when there are two exactly equal tasks. Even if they aren’t spot-on equal, though, this truth applies everywhere and everywhen: the time you spend deciding between the two is wasted.
Don’t spend time prioritising, spend time doing.