I have a differential signal(a +/- signal) to measure. Can I use a single ended probe to measure it and then just multiply whatever I read by 2 to get the actual amplitude?
In general, no. If you have a differential signal, use two probes to measure both "ends" of the signal and compute the difference between them. Most oscilloscopes will do this for you.
Usually the scope's input is referenced to earth ground so if you put your probes across the + and - signals, you will be shorting - to GND.
You can use 2 channels on your scope to measure + and - signals with separate probes referenced to ground and then use the "math" function on your scope to subtract the two like everyone else is mentioning here.
Alternatively, you could use an isolation transformer if you only have 1 channel available on the scope.
Hope this helps.