Crank threads stripped - both pedals fell off

Those cranks arms are gone/dead and need to be replaced. The pedals are steel and the crank aluminum. You should grease the threads so the metals do not fuse.

The only thing you can do that is safe and reliable and sane is to get a new crankset. You'd need a helicoil or something similar to do the repair reliably and this would be less strong than a new crankset and cost more than the new crankset + installation.

Next time, make sure everything is greased and tightened properly.

The threads are completely stuffed. Do not just screw the pedals back in and do not rely on loctite or similar compounds. Pedals falling off is at best inconvenient, at worst can lead to crash and serious injury.

Easiest option is to replace the cranks. Its not a big job but parts cost might mean a repair is a better option for you. The bottom bracket will probably be a square taper, so you need a special tool, or the LBS can do it in a few minutes.

The only other realistic option is to insert a thread insert (Helicoil) thread repair into the cranks. The left hand crank is a left hand thread. (Park tool have instructions)

First let's think why this occured.

I believe this is what would happen if one used the left pedal for the right crank and the right pedal for the left crank arm. You see the pedals are made so they screw in with the same rotation as the pedal spins (on forward motion of course). So as you press the pedal the pedal, even if not screwed in correctly, it would go inwards rather than outwards and fall off. Assuming you did not engage into crazy back-pedaling (en route or as the bike was standing which I do not think you did) it would only make sense that these pedals were placed right-left reversed. That would have the effect of what you are seeing i.e. the more you pedal the more they come off and eventually fall off. (Note your FSA Tempo crankset is alloy as I found online so the thread would not just go bad without special force as the one described above)

Now to move on (assuming this is what happened) there are two possibilities:

1- the threads on the crank arm are damaged and cannot hold the pedal (you say pedal seems ok) OR 2- the threads on the crank arm are not so damaged and can hold the pedal

Firstly, avoid using anything out of the ordinary for your pedals. Even if nothing else you will probably end up with a permanent pedals/crankset bond.

To find out where you are at just place the pedals correctly this time (and using proper grease) tighten them but not with too much force. Examine and if it looks ok take your bike out for a careful spin on a level road. If it looks and sounds ok try a bit of uphill progressively. If everything stays in place for all the uphill you tried and no funny sounds (even ticks) can be heard when moving then you can start to carefully continue your rides. You probably saved the money for new crank arms.

If not you only need the crank arms. Make a note of your BB-to-crank arm interface (most likely square tapered), BCD measurement and the length of the crankarms and hit ebay or LBS to get them they should be quite cheap if they are not carbon and are used. Avoid no-names. If price is ok you can get the whole crankset (in that case note your bike frame BB type e.g. BSA/road).

Always be careful when riding especially a mended bike. Ride into different situations slowly and cautiously.