Joined faces can't be cutted

I’m newbie in blender. I’m using the 2.63 version.
My objects are getting very subdivided so, I like to join the faces I think I will not use.
So I select two faces, and hit F key. the faces are joined in only one.
But when I select this face and subdivide it, nothing happens, I can’t do any kind of cut on it, you know, knife, loop cut, and so on…doens’t work anymore.
How to join faces with this kinds of cuts working on it?

Can you post an example screen shot?
I would select the edge between the faces (in edge select mode) and then remove it (X) using the option dissolve. When you had previously two quads you will get now an N-Gon with 6 vertices.

1 - Here I have a simple cube selected, I pressed W to subdivide it.

2 - Here is the cube subdivided. Everything is okey.

3 - I rotate the view to see it better, I selected the 4 quads of one side to join it in one face.

4 - Later, I hit F to join the quads into a face.

5 - So, I will try to subdivide this side again.

6 - Nothing happens, and the center point of the other faces come more close.

How can I cut this??

For now, cut it with the knife tool.

The problem is that it’s an ngon at that point, and you can’t subdivide ngons.

Knife dont work too… I can’t cut it not matter what I do.
How can I join faces I can cut after with knife and loop cut tools?

If you don’t want to use the knife (when I repeat the actions above the knife works perfectly to split the face) another way is to select two oposite vertices and press J to split them, with this new edge selected W / Subdivide, then select this new vertex and the top vertex and J again to split, repeat for the other remaining face.

You are right, I can cut it with knife tool, sorry for my mistake.
But it can’t cut with the loop cut.
Now I know why.
Thanks for the answer.