two 15 amp outlets to pigtail 30 amp?
No, you absolutely cannot do that.

For starters, they will be on different legs of your hydro connection. You will create a short circuit.  There is 240v between the hot wires of those plugs. Even if they were on the same leg, it's still not allowed or advisable, because there is no way to make sure they share the load evenly.  Either way, you'll end up damaging your circuit breakers by continually tripping them, or start a fire.

What you need is to run a new 15 or 20 amp circuit with appropriate gauge wire (which might be thicker depending on length of the run) outside.  Since you know you'll be using long extension cords with it, I'd add the length of my longest extension cord when calculating the length of the run, and see if that makes you need to upsize the wire further.