In push pull resistance welding two transformers are arranged opposite each other such that the opposing electrodes are opposite polarity and from the opposing transformer, as shown below. This arrangement is advantageous for making many spots in automated systems. All cables can either be above or below.
The question is how many SCR’s are required and how are they sized? In the above one SCR would control both transformers, fired at the same time. Therefor the sizing of the SCR would be based upon the total current draw and sizing of the two transformers making the four welds.
To size a transformer see article: “HOW TO SIZE A TRANSFORMER”
Reference: RWMA Manual- Fourth Edition