Connecting transformers in parallel is sometimes a risky business,

Some big power distribution transformers are sometimes connecting in
parallel with some external inductance to reduce the imbalance.

Take a separate smallish transformer with a centre tapped low voltage
secondary winding. Connect the centre tap to the load and the two end
of the small transformer to the respective hot terminals on the main
transformers. The primary of the smallish transformer is not
connected, but can contain quite voltages.
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Question : With a load having 2 connections , with the centre tap
connected
to 1 connection of the load ,where to connect the other connection of the
load ?

To the "cold" end of both main transformers, which are connected
together. You have to connect the primaries in such a way that the
"hot" ends of the secondary of each transformer is in the same phase,
thus the voltage of the hot end of each secondary is nearly equal and
the centre tapped choke will take care of the rest.

In three phase systems, connect the star points of the transformers
together, which is also used as a return for single phase loads. A
balancing CT choke is needed for each phase and also in this case you
have to observe the primary polarity when connecting the primaries.
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Tnx Paul , for the very useful info ,which I have filed for future
reference.

Frank GM0CSZ / KN6WH