Are scissor jacks safe to use instead of jack stands - for getting underneath the car for a short while?
In a pinch, I have done this...scissor jack from one car to lift...use the other as a failsafe...
...it's a TERRIBLE idea. If the car shifts in any kind of substantial way, you'll be as likely to tip both jacks as you are one. The only reason I've done it is to lift ONE wheel off the ground, and do something in the wheelwell- not actually putting myself under the car.
It's just that one shitty point of support is inferior to two shitty points of support...but it's still, well...you know...shitty.
I will say that I don't object to the idea of a scissor jack for lifting, especially if it's a particularly beefy one...but always support something with proper stands before you get in a position where you could be injured/killed.
I've got ones from junkyard SUV's are are decent for throwing in your trunk with a pair of stands, rather than trying to lift in a 100lb floor jack. Or depending on an el-cheapo small hydraulic jack.