I know there is a new catch phrase around: "There's an app for that", but guess what...... not EVERYTHING is done via apps.
Sometimes, we still need to learn how to do something and need to make an effort to get to the result.
Other times, there really IS an app for it, but doing it yourself the "hard" way gives far superior results.
Learn how to use a proper image "editing" program.
Adobe Photoshop, PS Elements, PaintShopPro, Gimp or even Photoscape (the last two are free).
Using a website is fine if you only want to do a few photos, but resident programs are the much better option if you have more or want to do this regularly. If it MUST be a free website, try http://pixlr.com/ or http://www.sumopaint.com/ or http://aviary.com/ .
I don't know what you want to combine or what you mean by "face onto each other", so I'm going to just have a guess that it involves people.
In order to "combine" two photos, both must be suitable candidates.
It is no good if they include people who have bits chopped off, or who are covered by something else, etc.
Make sure both shots were taken from the same angle and under the same lighting conditions.
If angle and light don't match, it will never look anywhere near believable.
Then you need a GOOD image editing program (such as Photoshop, Elements, Gimp or PSP). Use whatever selection tool is most appropriate for the job, isolate the person, then copy that person onto the other image, on a new layer.
Blend the edges, and use a drop shadow to make it look more realistic.
Projects like this are very easy to do if you don't care how good the end result looks.
If you want it to look real, then you need to do a LOT of learning and practicing, these skills don't happen in a few hours.
Also have a look at my multiplicity example here, complete with mini tutorial: http://www.flickr.com/photos/beetle63/5215060539/in/photostream