Combining heating extension and old part of house

Hi all,

So i have had two different plumbers with two very different opinions - so i really hope someone could add some clarity - thanks in advance

Many years ago my house had an extension - the downstairs was given planning permission the upstairs wasnt.

Despite many appeals the upstairs was never given planning permission.
reluctantly i decided to extend just the downstairs.

The situation currently is i have a very big downstairs but a very small upstairs (two average bedrooms and a boxroom).

Ultimately we have decided to incorporate a ensuite-bedroom downstairs for my elderly mum - sacrificing one of the three living rooms. Luckily the mess/building work is confined to just the one room.

so herein lies the problem

When the extension was done our plumber at the time kept the old part of the house on the old boiler system (15mm copper pipes) and the new extension on a new boiler (22mm copper pipes).

our home has two climate zones, so the new extension downstairs is warm and cozy, but the minute you head upstairs you walk into antartica and vice versa.
Obviously far from ideal

Iv asked two plumbers if it would be possible to connect all the radiators onto just one boiler 8 in new extension and 7 from old part of the house

First plumber said yes possible - but the boiler would have to work, pump harder - but do-able.

Second plumber flat out said the old part of the house wont get enough heated water pushed through radiators hence cold spots.

The current situation - is far from ideal. Just want a heating system to heat the house as a whole.

Can anyone help with some advice

Thanks in advance