You're trying to combine "parallel DC power supplies." It's not as simple as Lego, though, because typically one supply is a few millivolts higher than another, which drives the second one in reverse. If you're lucky, nothing catches on fire besides that power supply which, to its astonishment, has become a power drain.
With a few diodes rated for that current, you could solder up a circuit. Or you could do something even fancier. But for the numbers you need, it's much simpler and faster to just use a household 9V battery. They can supply 500 mA for about an hour. Or buy or scrounge a 9V wall wart rated for your 280 mA.
Edit: you can safely parallel-connect multiple 9V outputs of Voodoo's Pedal Power 2 Plus, because those outputs are isolated from each other and probably have other safeguards because the unit is popular despite its price exceeding $20 per output. It's clearly not from a fly-by-night Pacific factory.