Hi BuckSkin,
Nice idea to combine the 2 photos but if you are aiming for realism, unfortunately the merged photo doesn't quite do it for me.
Taking elements from one photo and putting them into another photo is sometimes straight forward and sometimes takes a bit of extra tweaking to make the composite photo look realistic.
To my eyes, it is very clear that the 2 girls on the sheet have just been dropped on top of the beach photo and were not there originally. The tell-tales are 1) the 2 girls are obviously stretched horizontally 2) the brightness of the 2 girls is far too high compared to the lower background brightness 3) the sheet does not "blend" into the sand.
If you would like to add more realism to the composite, some things you could do include:
1) compress the girls horizontally back to their original proportions.
2) Burn some light shadows between the sheet and sand
3) Clone some sand over parts of the edges of the sheet to help create the look of the sheet lying on the sand.
4) Blur the edges of the sheet slightly to soften them.
Also, the colour of the sand doesn't look natural. On my screen (calibrated) it's a dull bluish, redish, whitish, greyish mix. This is similar to an incorrectly exposed snow seen where the snow turns out a bluish, greyish off-white colour. When metering a scene, the camera has no idea at all of what you are pointing it at. It doesn't know if it is pointed at a beach scene, a portrait or anything else. It just measures the amount of light coming from the scene and then tries to convert it to an 18% grey. For example, if you correctly exposed and photographed a pure white wall or a pure black wall, both images would be just a constant middle grey colour. So given this, when metering a predominantly white seen (snow, beach sand or whatever) your camera will try to make the image an 18% grey and so your image will be too dark. Contrary to most peoples' intuition, when metering a predominantly white seen you need to add a little extra exposure to counteract the camera's tendency to convert what it sees to an 18% grey. Vice-versa when metering a predominantly black scene.
For what it's worth, the way I meter a scene is:
1) I use spot-metering for all scenes.
2) I look for something that is 18% grey or a neutral colour in the scene and spot meter it to get my exposure. If I meter correctly an 18% grey in my scene, then the rest of the scene should be properly exposed in the vast majority of cases. If I can't find an 18% grey in the scene, I look for a neutral colour I can meter. Sunlit grass is what I often meter in outdoor shots which is a good 18% grey substitute.
Hope this helps 
