I have a fusuma paper that is 180[cm] high, but my door (panel) is 250[cm] high. Is it doable to place two "sheets" of fusuma paper on one panel (without us noticing that there are two different sheets)? The idea is to cover the whole panel.
You can - and people do use two sheets of washi (shojigami) on a fusuma door. However, this is often because of a repair or as in your case, the remainder of a single sheet won’t fit.
The seam is almost impossible to hide because you’ll have to overlap the edges of the paper. It won’t be visible from one side but very visible from the other.
Your home center should have rolls that’ll fit most fusuma doors so if you really don’t want a seam, get a larger roll. The new synthetic washi is much more tear and UV resistant and is worth the extra money, even accounting for the special double sided tape you have to use (normal rice glue doesn’t work on synthetics).