linking two radiators together

I am in process to removing the wall between our kitchen and dining room.

The dining room has a double rad 620x 1450 and the kintchen currently a 620x500 double however according to btu calculation the kitchen rad is too small and should be upgraded.

As I 'm going to remove the wall and make one big room ( 6m x 4m x 8ft ceiling height-kitchen diner with back door 2 exterior walls and 3 modest sized windows)I plan on moving the existing dinning room rad onto the long wall and joining another bigger rad to replace the current kitchen rad onto it.

whats the best way to do this, I have been given two options by family so far.

opt 1, using the existing dining room pipework use tee junctions and have two independant rads running of the current circuit??

opt 2. remove the top part connection from the rad and link them both directly together via piping to make one continuous radiator with one feed and return??

Whats the best option. The existing pipework is 15mm if this helps, is there a max btu for this pipework supplying rads.

i would say option one is the most flexible for instilation /repair/decorating and heat output

Btu limit is about 4kW
Put them in parallel.

are u on solid floors ? is that why u ana join the two rads together, if u choose to join em together do it at the bottom but i would pipe em seperate if possible..