I have a heat-sink and I want to put two FET's on it. I know I have to insulate it so the metal showing on it doesn't connect and create a path. Is there any good everyday material that would work? (ex. plastic bag) My packaging is TO-247 (irfp250/450) and I found 1 insulator laying around, but not 2 for both of them. So do I need to put the insulator on both MOSFETs or is one good enough?
Depending on the MOSFET package, mica washers are an option.
A common through-hole package for MOSFETs is the TO-220, where the metal tab looks like this from the back:
Edit: OP clarifies that their MOSFETs are TO-247 packages, for which the TO-220 insulators can be used - or TO-247 ones possibly, though those don't seem to be sold as a distinct product much.
For these, mica insulator kits are available at extremely low prices, containing the washer itself, a nut, bolt, spring washer, and an insulating sleeve to isolate the bolt from the metal tab:
Insulation is preferably done for each of the MOSFETs you are attaching to a heat-sink, to avoid undesired short circuits.
You should also use a bit of heat-sink compound between the mica and the metal tab, as well as between the mica and the heat sink itself.
The modern alternative to mica and grease is the Sil-Pad products from Bergquist and others. These are not very expensive and they're a lot more convenient to work with.
For very good thermal performance and isolation, go with Bergquist Hi-Flow.
It's phase change material that goes liquid-ish above 55C, filling whatever voids are necessary, plus it has a Kapton insulator in the middle to give you good dielectric strength. It works best with clip-type connectors that hold constant pressure, otherwise you need to pre-bake the heatsink and torque down screws a second time once the material flows.
One more alternative to put out there: They are rare but at least one supplier makes MOSFETs with electrically isolated tabs. IXYS sells these under the marketing name "ISOPLUS" but you can find more by googling "MOSFET ELECTRICALLY ISOLATED TAB".
These could bolt directly to the heat sink and, for anything being manufactured, could save a lot of time.
So do I need to put the insulator on both MOSFETs or is one good enough?
Depending on what you are constructing, you may as well isolate only one of the transistors.
It is up to you to decide if you are OK with the radiator having the drain voltage of the other transistor in terms of safety and reliability.