Let's start with the two mics question: you should only use two mics if you're able to record them in separate channels.
IT IS NOT A GOOD IDEA TO USE A Y CABLE TO MIX BOTH MICS WITHOUT A MIXER.
Why? Well, if you're using two microphones you be getting a STEREO recording - and that's awesome! You can work with Mid/Side eq to balance the sounds and even set the mics in a binaural set up (more information en.wikipedia.org/wiki/Binaural_recording - listen to the audio samples with a pair of headphones to understand what it is)
But then, why shouldn't you use a y cable to get a mono mix of two mics?
Depending on the position of the microphone they may cancel each other, you might get phase problems and even a lot of artifacts provided by noise captured from both mics.
This excelent article from SHURE(PDF) has some information about recording:
On pages 21/22 there's some application of stereo mic recording and on page 29-31 there's some information on phase canceling, comb filtering.
Now, that covers the mic placement and stereo recording part of the question.
The outdoors part is a little bit more complicated.
You should read this short article from Soundonsound
The main recommendations they make are:
1.) Fur protection on the mic.
2.) Shotgun directional mic.
The fur is an essential thing to do - the shotgun part (using a omnidirectional mic) is a little bit more complex - there's is no easy answer about what you should do. Take note that the Shure article covers different types of recording positions for microphones, depending on what the situation is.
There is also this awesome article about field recording.
It's the opposite of what you wanna do, but still good information.
NOW WHAT WOULD I DO IF I WERE YOU:
Buy a fur protection for your mic and go to a windy place and record something, check if the fur is enough for recording outdoors without wind noise.
Get a cellphone/boombox/guitar+singer and try to record it/he/she.
Check the recording later, see how is the wind affecting the recording - is a simple filter enough to remove the artifacts?
And more importantly: when you say 'group of people' and 'spread across the park' - how many people are we talking about and how spread are they?
The main problems you'll encounter are obvious: shotgun mics will not record everyone and non directional mics will capture wind noise. I have little experience with outdoor group recordings - the most I've done was capturing a voice/acoustic guitar performance on a windy park. I used a cardioid mic with fur coat for the vocals and recorded the guitar DI.
But then again I cannot stress this enough: Test your situation. Just go around and experiment with what you got.
