is it allowed to weld T-piece with T-piece?

Yaroslav,

Looking at the piping arrangement here there's absolutely no issue with butt welding fittings together of any sort on typical SS or CS specs and the like.
To reiterate 11echo in other words:
In fact all good piping designers should strive for this type of configuration because of cost as well as design.
You are saving in shop labour here reducing welds on time spent welding and material costs with the extra pipe piece between.
As a designer coming from a piping fabrication and installation company back ground this is how we were always taught to design taking into account the fab shop and associated extra costs. Too many designers are not aware of this and will add in extra fittings and welds when they are not necessary.
I understand there will be cases where there needs to be pipe between fittings in high purity situations to allow the weld head in to produce the weld.
Personally if I took a design with poc pieces of pipe between fittings I would alter and produce the iso's butted together for reasons giving.
This is what I instruct my team to do also where applicable as good piping practise.

Hi Guys,

Newcomer to the site, I'd just like to add something, drawing from my Pipefitting back ground.

Agreed Shay and 11echo, cost must be at the forefront of all designers work, but i think what Yaroslav means when he said " weld between two T-pieces will be wider compared to weld of T-piece and pipe" is refering to the asthetics of the finished weld. We all know the the most important factor in butt welds is the matching ID (inside diameter) making it look pretty is then up to the welder.