Would it be possible to use one FESTO 24v double solenoid to actuate 2 identical pistons simultaneously? We have one 24v solenoid but our AMAZING treasurer bought a 12v one for the second one. We don’t have time to get another 24v solenoid so I was thinking it might be possible to just use one, but I wanted to just make sure.
yes, it is entirely possible. Use a T-splitter in your pneumatic tubing coming out of the solenoid to each of the pistons. To get them to fire the same time, make sure that the tubing if of equal length after the T-Splitter, or one will fill before the other. We use this a lot (and acutally twice this year!) for pistons that we want to fire at the same time.
In the future, send your treasurer a link of the exact thing you want. If they don’t buy it off that link without asking you first, then they shouldn’t be your treasurer. 
Yes you can.
McMaster allows you to build and forward complete orders. Never had a single mix-up (yet) using it. The treasurer just has to click ‘order’ and fill out the correct information.
If you end up needing a second solenoid, the 12v one can be controlled via a Spike relay.
Where is your team located ? Perhaps another team could give you one to use. Then order an exact replacement and give it to them. Or maybe you could trade your 24v for another 12 or vise versa