[R] 2 images on one plot

[Petr PIKAL]

Dear all

with image I can plot only one set of values in one plot. 

Do somebody have any insight how to put those 2 matrices into one picture 
so that in one cell in image picture are both values from mat[1,1] and 
mat2[1,1].

mat<-matrix(1:4, 2,2)
mat2<-matrix(4:1,2,2)
x <-1:2
y <-1:2
image(x, y, mat)
image(x, y, mat2)

The only way I found is to mix x or y for both matrices  let say

xm <- sort(c(x,x+.5))
matm <- cbind(mat[,1], mat2[,1], mat[,2], mat2[,2])
image(xm,y,t(matm))

which lacks of elegance and is rather complicated when considering matrix 
with more rows and columns.

Looking forward to any hint.

Regards
Petr